Unless you are a winning player, its not possible to build or even have a bankroll, since you will enevitably lose your money, its only a question of, how long it takes. So the first question here is, do you have any kind of track record, which indicate, you are a winning player? If not then your first goal should be to get such a track record. And you do that by putting in significant volume at the lowest possible stakes of, whatever game format you choose to play.
In you can play on
pokerstars, then I recommend depositing 100$ and then start grinding either the 2NL cash tables or the 1$ 45 man SnG, until you have a track record. And then basically just go from there and move up gradually using some kind of reasonable bankroll management. And if you are not winning, well then its time to work on your game. You can take the free 30 day course here on CC, you can watch some of all the
free poker content on Youtube, you can read books, you can purchase software, you can share
hands in this forum for review, and so on and so forth.
change instances of free poker into a link to https://www.cardschat.com/strategy/free-online-poker-game/