It purely depends if you think you can learn the game better by just taking things one step or if you prefer to just get thrown in the middle of everything and try to figure out how it all works.

Either way, one tip I'd recommend for new players is to play your first game without any AI opponents (I.e. you are the only player in the game), as the tutorials really don't explain how nearly everything works. A lot of it you'll just learn as you research and build things, so this lets you experiment without any real risk of losing. If you'd prefer to start against the AI right away turn the pirates off, they are good for combat experience when you're learning by yourself but they tend to give new players a hard time in a real game.

I started a new game and decided to go progressively. 1 game of each before going to trinity.

Epic spaceship battle !

I tried two normal games of sins, but yeah pirates drain so much of my attention I neglect my Easy computer AI ennemy...and when I'm finnally ready to get more than 2 planet and 2 asteroid I dont have enough ship to take on a planet. lol

Hard game, so much fun thought.

Freaking pirates killed all my colony while fighting the ennemy

Here's a tip: you can colonize the planet even before the militia are all dead, and then start building turrets and repair platforms to help fight them.  Only the "Krosov Siege Frigate" unit is capable of bombarding your new colony, so if you quickly knock out them you're good to go.

The Krosov is actually vulnerable to the scout of all units.  A pack of a dozen scouts can be used to very quickly snipe out the Krosovs and allow you to colonize.  By jumping in your capital ship first, it will aggro the militia and very few will attack your scouts.  It can be a little tricky to sneak in a colony frigate to actually lay down the colony, but if you're using a capital ship with such an ability, it's trivial.

As for defending against pirates (or defending in general), repair platforms are your friend.  They're really inexpensive, and vastly increase the longevity of your units.  Be sure to always build them in pairs, since they cannot repair themselves.  Building a frigate factory nearby can be a good idea, and this makes a very solid and cost-effective defense since you'll constantly be building reinforcements during the fight.  One of the big problems with hangers and turrets is that they don't do anything if the enemy isn't attacking, but a frigate factory / repair platform combo gives you a nice place for your forces to regroup even when you're on the offensive.

I don't know why the devs made pirates as they are, as they can cripple newer players.

That said, it isn't that hard to defend against them if you know what you're doing: a few turrets (gauss defense, for TEC) and a few repair bays should take care of most of them.

The AI is pretty good about outbidding you and forcing you to take the pirate attack - this is especially true on the higher difficulties. It's very annoying.

You could just turn the pirates off. That's what I'd do, even though I'm relatively experienced.. They're too chaotic and annoying for me. Those who play online almost never play with pirates anyway.

Although decent players can repel pirates reasonably well, I tend to think that the investment required is more than it should be, in Diplomacy at least. A capital ship out of action from the 17-25? minute mark, plus maybe 2 extra repair bays, plus some more for turrets if desired, is simply too much that early in the game - if the players are playing relatively competitively. Of course most of such players play with pirates off, so this is a moot point anyway, but still.

And all this can be done to a player at a very low cost (250 creds possibly, maybe even 0 often) and can also simply be a result of bad luck (not increasing bounty at the precise second necessary).

When a good opponent can have 2 carrier capital ships knocking on your doorstep at 15 minutes, the addition of pirates can mean the loss of a front-line planet in 1v1, which could make the difference between a win and a loss depending on planet type and placement.

The all-or-nothing is a problem, but small refunds are possible, in a way, if you anticipate it, and if you get lucky... by attacking the other player with whatever meager fleet you can summon up at 15 minutes (caps, most likely) and killing a few of their ships or strike-craft for a little bounty reward. (hopefully the enemy has close enemy frigates or SC)

In fact, this probably wouldn't be a horrible strategy for either player, whether they win or lose the bid, so they can get a few credits back... although in some cases it could well be better for the cap(s) to be doing something else instead, like fighting militia.

Thanks for all the help here,

but now that I turned the pirate off, I'm playing against an easy CPU. I think I managed slowly to win battles and defeat some capîtal ship with taking maybe half of a small map (14 planet)

then I decide to move on the suposed weaken easy cpu and I see a fleet double of mine (70-80 vessels)

wtf ? lol

I guess I'm playing the game too slowly, galciv 2 style.

Must I colonize lot of planet fast or must I devellop a few of them before ?  Where to build what and where to build defense ? I guess intersection point are the best to hold a defense right ?

I've been handling battle pretty well but for my 3rd game I started I feel like I'm not able to catch up the pace of the game.

And I'm a veteran gamers...I'm no noob to this kind of game here. so I'm I doing something wrong ? 

Yeah that's probably the thing. In Entrenchment this is a little more viable, but only because the AI sucks at attacking defenses, not because its a good strategy. Early game you should try to get as many neutral planets as possible, and in general only start to develop trade and high population upgrades once you can no longer expand easily. Once you have a decent sized Empire you can turtle against the AI if that's your play style, but for the first stage of the game you should be quite aggressive about expansion.

The good thing about original Sins is that it forces you how to really use your fleet well as, both tactically and strategically, as your defenses are fairly weak, forcing you to beat the AI without the "crutch" of starbases that many casual players use to beat the AI in the expansions. Not that it will get you to the multiplayer level, but better than the expansions.

These "bidding spirals" are certainly quite useful.

If you get bored with single-player, I highly suggest trying your hand at online games with humans instead. Humans add a much more strategic dimension to the game: although they're harder, humans are much more enjoyable to play with, in my opinion. There's much more tactics and strategy involved.

Though mods are certainly an interesting creative exercise.

Are you developing your planets out of the red? (if you don't develop planets immediately after you colonize, you lose a lot of credits!)

Yes, after accounting for development costs, you should generally be colonizing as fast as possible. Try using the turret and scout trick Darvin described: you should be able to get a new planet at least every 4 minutes, until you have 5 or 6 planets.

It also helps a whole lot if you can research trade ports, and build a few of them in a chain; trade ports increase your credit income rate. If you can do that, then you should be able to support a pretty decent sized fleet by the 30~ minute mark in the game. Build frigate factories on your front line world, build a fleet (composed mainly of LRM or Assailants or Illuminators) and attack the enemy! If you're playing against Easy this should make winning the game relatively quick.

Generally, you shouldn't be researching anything except for the bare essentials (ship types, planet colonization), and you shouldn't be building much of anything on your planets except for extractors, trade ports, and the few necessary military/civilian labs.

If you do this, then it shouldn't be difficult to build a fleet much more massive than the Easy AI and steamroll it.

If the AI has a large fleet like that, it'll be mostly combat-oriented... the solution would either be to have a starbase, or to have the correct countering units, or to overwhelm the enemy with numbers.

Not sure about the trade patterns but the pirates become a whole lot easier with Entrenchment. Starbases are the death of pirates. Diplomacy made attempts to compensate but never truly made a successful attempt aside from the über pirates of death, which have been nerfed down to something "manageable". Some mods (Distant Stars being one) made better attempts (aka capital ships with anti-structural abilities) but the devs refuse to use those methods.