First of all understand, what a bankroll is, and then decide if you even need one. A bankroll is an amount of money, which makes it reasonably sure, a winning player dont go broke due to variance. This is important for professional players, because if they go broke, they lose not only their money but also their source of income, which is obviously very bad. However there are two important points to notice here:
1) Losing players cant have a bankroll, because if they continue to play long enough, they will always go broke.
2) You dont need a bankroll, if you play for fun and can take the money for playing poker from a regular source of income like a salary.
The way, you ask this question, you are almost certainly new to poker, which mean, you have no track record to prove, you are a winning player. What you really mean, is therefore, that you are going to start playing poker.
Then first of all define, why you want to play poker. If its mainly for fun, then make a budget and fine the games, that give you most fun. If you win anything, thats cool, and maybe you dont need to deposit next month, but winning is not your main expectation.
If you want to play for profit, then invest in software like at minimum a tracker and start practicing with games like 2NL and 5NL, until you have a track record to prove, you can beat those games. That could be something like 50.000 or even 100.000
hands. Also spend time studying and reviewing your game to become better.
At this point maybe try SnGs and then decide, if you want to go the cash game route or the MTT route. And all the time remember, that your goal is to get better, not to "build a bankroll". If for some reason you had to withdraw winnings, or you want to try another
poker site, then dont start all over at 2NL again, if you are already beating 10NL or 25NL. Find money from outside poker to allow you to play those games, where you have the highest hourly winrate, whatever these are.
