start with the rule of 4 and 2
examples
you have a flush draw on the flop..holecards Ace clubs/Ten clubs..flop 8clubs/2clubs/3hearts...the chances of hitting your flush are 4x9=36%..9 clubs are left in the deck discounting your clubs and the flops...
then 2x9 on the turn..=18%
so count your outs and use 4 and 2 and combine the results with the odds you need to continue..
then when you've understood that bit, look into implied odds.