You could start with as little as £50 (I appreciate that is quite a lot for some!).
The more cash you have to start with, the quicker you will build up a pot of money to use, and so more profits early on.
I started with Team Profit which is free to join.
They have very helpful videos that explains it all clearly. They also suggest how to start with whatever amount you have available and guide you onto the next offer - the minimum they suggest is £10, but I think this would take a very long time to build up any profits with this.
I found it quite time intensive initially for the first few weeks, whilst I was learning how it all worked. Once I became more confident, I was able to just spend a few hours each week to keep it ticking over for a regular income.
I am very cautious and risk-averse, and never gamble or place 'real bets'. If you have an addictive personality or are in any way tempted by gambling it would not be for you.
https://www.teamprofit.com/
A couple of tips if you want to go for it:
Set up a separate current account with a different bank to use so payments to bookies are not visible on your regular bank statements if you need to show them.
Set up a separate email account just for MB, so you're not inundated in your regular account, and can spot new offers arriving easily.