Sit and Go Tournaments tips
Start small. No matter how much money you have, whilst you are still a beginner, try to stay to the small tables. You are learning all the time. You may think you know a lot about poker, but almost certainly you will be playing someone more experienced.
Use the oldest tricks in the books. The beauty of online poker is that you cannot be read as easily, also it means there are many people playing who *think* they are good poker players. Use this to your advantage. One of the oldest tricks is to check-raise. If you manage to flop the nuts (the best possible hand on the table), you can use this to your advantage. If you check, it shows the other players that you are not confident in your hand. You can even check again on the turn. This also gives your opponents a chance to try and bluff their way into buying the pot. Your ideal situation is for someone to put in a large raise towards the end, trying to buy the pot and then you can simply call them all in. This should be a contribution to your stack.
If you find yourself in the position described above, namely you have a mediocre hand, but are waiting for some kind of draw, you need to be careful. For me, in this type of situation, I would hope that a player that plays before me puts in a substantial raise and makes my decision for me (to fold). However if there is only a small bet on the table and I am towards the back of the table (playing last), I might call the bet to see the flop. If I don't flop anything helpful, I would probably fold.
In STT's (single table tournaments), a common mistake for beginners is to get too excited. Getting too excited basically means entering too many hands. A beginner who enters an online STT should sit back for the first 3 - 4 levels of blinds, only playing premium hands (AA, KK, QQ, AK). This enables you to get a feel for the people around the table and will ensure that you do not go out too early.
Be aware of your position in the table. As the dealer moves around, be aware if you are early or late in the order of play. The later you are, the more influence you have on the size of the pot. This can give you a slight advantage and you may be willing to raise with slightly weaker hands. If you are early in the table and you raise with a weaker hand, there is a lot of oppurtunity for someone to re-raise you, probably causing you to fold.