Say i use a personal check to purshase goods online does the money in my chec. acct get taken out that moment?

Okay heres the deal i want to buy stuff w/ my checkin acct online but i really dont have enough money right now in my account, but i will be putting in a lot of money on wednesday.. so my question is does the online merchant automatically take out the $$ or does it take a few days? when will it - my acct?? confused. help me out!