ok, I'll speak from experience here, which I have a lot of , with edibles.
the worst that can happen, you can get so high your whole world is either spinning or very intense have to sit in one place or you might puke or you might just pass out. don't take too much start low work your way up.
the other worst thing that can happen, is nothing, which means you messed up your method somewhere with either not enough heat, too much heat, not enough fat in the substance you are using, not enough weed, etc.
as far as how much to use.... you have to figure out your dosages.
the average edible high takes about 4-6x more weed than how much you would use to smoke (usually). so you figure it out, do the math. this is as much info as I can give you, if you need anything else just ask and Ill try to answer.