I haven't watch Judo much but i have seen them wrestling on the ground sometimes. Do they ever start on the ground? or do they only wrestle on the ground after a throw?

I used to start on the ground all the time in practice. I'm not sure about competitions as I never competed in judo.

In matches sometimes I stand up to begin with then sit down and wait for them. But no usually you fight standing up.

Sometimes we start standing. Sometimes we go to the ground after a throw. Sometimes we start from the ground. My school mixes TKD and Judo, so it could come from grabbing a kick (take-down) or simply doing a hip throw from a punch attack. It all depends on the attack. The "wrestling" is each person trying to counter what the other is doing, basically, till one sumits (taps out).

In comps they always start standing or atleast that is all I have ever seen. I myself would take BJJ and then take judo as a complenment to takedowns.

well, there is matwork practice where you start sitting down back to back and they say hagi may and you wrestle.