There are basically two reasonable ways to play poker. One is with a budget, and the other is with a bankroll.
Playing with a budget
This is as simple, as it sounds. Decide how much money you want to set aside for your hobby, and then stick to this plan. This could for instance be $100 per month. To help you stick to such a plan, many sites allow you to set deposit limits, and in this example you will set it to $100 per month to make sure, you stick with your plan. So when you get your salary, you deposit $100, and then you simply play, what you want.
Its pretty obvious though, that if you buy into a $100 tournament, that is likely to be the only tournament, you play that month. And even if you play $10 tournaments, you are very likely to run out of money, unless you only play 2-3 of them per week. So to increase the chance, your money will last until the next salary allowing you to have more fun and learn more, you want to limit yourself to something reasonable. Like maybe not playing any higher than $2.5 MTTs. Losing more than 40 BIs in a month could happen, but its not that common.
However since the $100 is only a budget, you dont need to very strict with the limits. If you want to try a $5 or $10 MTT, then you just do it. But of course you have to understand, that the more, you do this, the higher the chance of running out of money before the next salary. So if you really dont want to take a forced break for 5-10-15 days, then you need to be at least a little conservative.
Playing with a bankroll
A bankroll is an amount of money set aside for poker, which will not be needed for other purposes. The difference to a budget is, that you are not willing or able to add to it every month. Bankroll management then means to manage this bankroll is such a way, that the risk of going broke (or lose a substantial part of it) is reduced to an acceptable level, which is often thought to be something like 1-2%.
However this is really only possible, if you are a winning player. And if you are new to the game and asking bankroll management questions, then you mostly likely dont have any track record to prove, you are a winning player. And in this situation your only option is to play with a budget. This is because, if your long term winrate is negative, then its only a matter of time, before your money will be lost.
But lets say, you eventually become a winning player, and from now on you do not want to deposit any more money, you only want to withdraw winnings. Then a reasonable plan is to not buy into an MTT for more than 1% of your bankroll, and to a SnG for more than 2% of your bankroll. A $100 bankroll does not make a ton of sense, but if for some reason, this is all, you have, then you should not play any higher than $1 MTTs and $2 SnGs.
Really though if you actually are a winning player, you might want to fix first, whatever it is, that prevents you from setting aside more than $100 for a poker bankroll. Maybe decide to add $50 to it every month. In which case you can play a little closer to the "playing with a budget" situation, because even if you lose half your poker money, you will have $100 again, as soon as you get your next salary.
