I played a little poker few years ago. But I'm very rusty.
And I want to pu a $20 into pokerstars and start playing.
My questions are:
1. Should I learn SitNGos or normal cash games NL Hold'em?
2. Should I install a HUD ? if so which one?
3. Do you have MUST articles to read ? I'm very newb. I only know the super basics like table positions and stuff like that
4. Any other recommandations would help
Hi Thisapear,
Welcome to PokerStrategy 
1) What format should you choose is totally up to you. Some prefer MTTs, others spinNgos, SNGs or cash games. I prefer cash games. And I think there is more study material available here for cash. The formats are a bit different and you should probably stick with one at first. So I would recommend cash but honestly it doesn't matter. you could hate cash games and it is ok.
2) Yes. If you can use a HUD on the site you are playing on, it will definitely give you an advantage. If you don't have one installed yet, I would recommend hand2note because it is free for limits under NL25 and it is probably the most powerful tool at the moment. But the most important thing is to learn to use a HUD properly. Learn what the stats mean and how big of a sample size you really need to make any conclusions.
3) If you are just starting out and you knowledge is quite basic, you need to study a bit. Poker is not a very easy game to beat. I would start with the basic articles at our strategy section. Those are a bit old but give you some kind of an idea about the game. Then depending on the format you choose, you have tons of material available here. If you go with cash, I would recommend watching confidant's beginner course and the follow up course. You could also watch anything from LemOn or from me.
4) You have a lot to study, take it slow. Don't overwhelm yourself by watching 10h of new strategies in a row and then try to apply them all at the same time.
You also start with a quite small bankroll. I would recommend having at least 25 buy-ins for every limit. That way you reduce the risk of going broke significantly. If you start with NL2 you only have 10 buy-ins. You can hit a 10 buy-in downswing because of variance. It doesn't mean you are a horrible player, so keep that in mind. Also know that when your bankroll gets smaller, it usually is tougher for your mental game. You start tilting. And tilting is probably the worst thing you can do. Keep your head cool 
Hey Thisapear,
1.) That was answered pretty well by la55i already. Every format can be played profitably, so the choice is up to you. It should be mentioned, though, that while the variance in SNGs is generally higher than in cash games, your losses in this format are capped, which can be advantage if you tend to suffer from tilt during bad runs. Whatever you choose, just keep in mind that not everything that works in a cash game will work just as well in tournament poker, and vice versa.
A starting bankroll of $20 is pretty low to take swings in either format, so I guess MTTs aren't the best option right now. Freerolls are always a good way to practice, though.
2.) I believe this depends on the format. In cash games, a HUD can really come in handy, assuming that you know how to use one properly, but in SNGs and tournaments, I don't think it matters all that much.
3.) If you choose cash games, I recommend taking a look at the Overview of No Limit Hold'em lessons. There's a similar overview for SNGs, and also a quick beginner's article to get you started right away. There's also a free tool available for download here called the ICM Trainer, which, while it's a bit rusty nowadays, allows you practice those decisive Push-or-Fold situations in SNGs. Just keep in mind that you're not allowed to keep it running while Pokerstars is open.
And if need be, you can always brush up on your Poker basics with these articles, some which are already included in the NL Hold'em overview mentioned earlier.
Oh, and one more thing: If you don't have an account at PokerStars already and plan to open one for this, be sure to sign up through PokerStrategy to get access to a tracked account, allowing you to unlock more content just by playing. Check out some of the offers over here, you might even be able to boost your starting bankroll that way.
Cash games though is tough for me, but i feel it's much easier and more realistic in the sense that you don't have to wait 6hours of play to make money like in the MTT. tried it couple of times, when i play for long duration i start losing but when i'm fresh i win and make the right decisions, that is folding when i'm suppose to, bluffing when i'm suppose to bla bla..
