Bankroll, how much do I need?
If you are a beginner or just looking to have fun, don't invest any more than is 'fun' to lose. Therefore, if you're comfortable blowing a hundred bucks down the drain, put in $100 and see if you can win with it.
The best players on the other hand are looking to consistently make money at a given limit. If you are one of these players, you should have 200 big blinds at the limit you play. For example, if you play $2-$4, you should have an $800 bankroll ($4 x 200 BBs). For $5-$10, your target bankroll should be $2,000.
***Please note that these numbers prevent you from blowing your entire bankroll because of one bad run but its only referring to "how much you need" to sit at an specific stakes table
As an advice as well, you don't want to invest more than 200 big blinds on a single table unless you've proven that you're successful at that limit.
Hope this helps,
USC :)