About the sit-and-go section of Pokerstars, you can play from the micro limits (under 5$) all the way to the higher limits (100$ and up). When choosing the limit to play, always respect the 2% rule: never invest more than 2% of your bankroll in a sit-and-go or tournament. One of the most important skills of a poker player is a bankroll management skill. You actually might be a pretty good player but without a solid bankroll management you are domed to bankruptcy .
You will also find several blind structures, the normal structure blinds will increase in intervals of 10 minutes. There is turbo and hyper structures which increase at 5 minutes and 2 minutes. I personally recommend not to play this structures because the high variance. Be patient, play the normal structure where your poker skills will come to the fore.
Another variable is the number of players, you will find sit-and-go’s of: 2,6,9,10,18,27,45 players and even bigger sit’s. Some players will prefer play smaller sit-and-go’s and others will play the biggest, that's up to you. Be in minding that the bigger the sit-and-go, more you can win but it’s harder to finish in the money. Be prepared to play a few hours in the bigger sit’s, for instance the 9 players sit-and-go may take from 1 hour to 1:30 hours.
This is the basic about sit-and-go’s at Pokerstars, good bets.