I started like a lot of people do - losing. After some time, maybe half a year, I was done with losing and said to myself, I either up my game or quit gambling. So I took 800,- Euro and that was my starting bank. I always had a pretty simple money management and I started with staking 3% flat. Only single bets allowed.
I still remember my very first bet as a "pro". It was a tennis WTA match, Bammer to beat Bohmova @1,55. Stake was 24,- Euro (3% of 800,-) and it won, which gave me a net profit of 13,20 Euro. And so it started.

As the weeks, months and years went by, my winnings were higher than what I needed for my expenses. I only ever cashed out, what I needed to live and thus was able to compound - my betting bank grew and with it the stakes I placed on bets. I stayed on flat 3% stake for a long time. And with higher stakes and good, valuable bets, I was able to make more money of course. So, I never had to sell anything or borrow money. I started with the 800,- Euro and build my way up from there.

Nowadays I still have a betting bank (i.e. money that sits on my betting accounts, Neteller, Skrill etc.) and I am using this capital for my bets. I don't play 3% flat stakes anymore, it's lower now. I was never a fan of money management systems like Kelly for example, just for the reason, that I can tell that a bet has probably value, but I am struggling to tell precisely how much value it has. So flat stakes is a bit for the lazy ones, but just suits me right. And you should always do things the way you feel comfortable, if possible.