There are many types of poker and many types of poker tournaments. But the most popular one is definitely the no limit single table tournament, otherwise called a sit and go tournament. The buy-ins for these kinds of tournaments can vary from $5 to hundreds dollars.
Let's focus on a particular single table tournament, the one that is played at a table of ten people and they give away three prices: 50% of the buy-in to the first player, 35% to the second place and the rest, 15% to the third place. All the guidelines below are written for this special type of tournaments, where multiple prices are given away, and if you are entering a tournament with only one winner, then all strategies should be made more aggressive.
Basically, at the beginning, you will have to try and see as many flops as you can. This will require checking the blinds to see the flop when you hold a very small pair like 4c 4h, raising when you have a pocket pair like Js Jh and also check or call when you hold cards like Js 10s.
In tournaments that have a buy-in that is very, very low, you can expect to see many bad player that will very soon loose all they have and this helps your strong hands, as they will pay to see them and this way increase your stack considerably.
Then, later on, the blinds will be around 50$-100$ and this is when you should try to take for yourself the blinds chips. You can do that by raising when you are the first to do that, in early position. You will call the blind and in this situation make a raise of around 300 - 400 dollars. In this situation, you can make people fold, or if you have a raise after you then wait until they check and then reraise, so you will make them pay a lot to see the flop when you hold a strong starting hand.
Once time passes the tournament will evolve and the blinds will start to be situated around 150-300 or even higher and this is the part where players will try to make all bids before the flop. Pre flop action is big now and you should take a look at your stack and decide what to do depending on its value. If you have a low stack you can not afford the blinds or staling them .in this case you should wait for the hand to give you one pot, so you can get back on your feet. You will do this by going all in with and ace and a high kicker, A and K for example is a good hand. If your stack chips are sufficient, then you can try a slow manner of getting them, just take your time and play big only when it is worth it and you can take it. If, however, you have lots of chips, then you should try and get players out of the tournament. Focus on weak players and try to get them bet everything when you are sure you can win. Be aggressive and be sure they will all be scared of you and they will rarely call your bids, but you have to find those hands that can give them confidence and then make them bid a lot so they lose a lot.