Basically very little ~ ~ Not enough to know much about it !
I have started getting in to, small bets on horse racing ~ ~ The problem is, you have to do a hell of a lot of boring research ~ ~ If you were willing to do the copious amounts of the boring research on small field races ( probably no more that 9 ~ ~ If there was one or two more they would have to be genuine no-hope-ers ) ~ ~ Maybe maybe maybe, you could make a few euros on the betting exchanges.
As I say you'd have to love doing the research , know a good bit about the mathematics of odds ~ ~ Know what types of races to bet on ( young horses can be a bit un-predicable, for example )
It is said ( well on the internet ) that only 1% or 2% of gamblers actually make money.
Another problem is the Bookies will block you, even if they just think, you are a Threat of being a Winner ~ ~ Even amateur me, was partially blocked by Bet365 because I did nearly catch them for €9,500 on a 7 horse acca !
Apparently the betting exchanges are not as quick to block you, but will take a bigger % of your winnings if you have consistent success !