Your bankroll is the amount of money you have set aside to play poker with. Some players can easily add to their bankroll from outside sources while others have no way to add if they lose theirs. If you fall into the latter category, it is very important to not play at limits that exceed your bankroll. One very important point you should know is that until you become a consistent winner it doesnt matter how big your bankroll is. The only thing that will matter is how much you have to lose. For this reason, the information below is written for you assuming you are a winning player overall.
The first rule is to not confuse bankroll and buy-in. A buy-in is the amount you sit down in a game with or the entry fee in a tournament, not the entire amount you have available to play poker with over a period of time. For example, you may have a bankroll of $10,000 to play 15/30-limit holdem. If this were the case you would probably buy-in for between $500 and $1,000.
I have seen bankroll size suggestions ranging from 200 times the big bet at the level you are playing at ($800 at 2/4) to 300 times the big bet ($1200 at 2/4). My recommendation, particularly for holdem, is to...