I have a 6 speed sears craftsman riding mower. I put in gear 1,2 and 3 and the mower won't move. I notice the belt for the pedal was not going into pulley. I moved belt but now not sure what I did. If you release the break pedal the mower stops. If you push the pedal to stop the mower, the mower goes but it drives very very slow. Not sure what I did

Check your purchase records to see if a maintenance contract was added. Sears is typically very insistent about this at the time of purchase & it is a great idea. If you have this, then contact Sears.
The belt is probably worn which caused the initial problem. It will need replacement. Other drive train parts may also be worn. Start with the belt.

What's your model number.