Start with what you can do easily. Pull the valve cover off and first visually look at the both rocker arms for that cylinder. See if they are loose, if there are broken pieces of springs, rockers, or? laying around. Then rotate the motor without it starting and see if the rockers move. Put your fingers on top or the rocker as it rotates and see that they move equally, check the others to get a feel for what it should feel like. In all likelyhood, if the valve is stuck, the rocker will loosen and move as the lobe on the cam moves away and will bind when it trys to move the stuck valve. If the valve is clearly not moving up and down, you will need to pull the head. You will need to know if the valve burned a hole in itself,if part of it fell on top of the piston and beat a hole in it, or even scored the side of the cylinder. With luck, you will find something broken on the top side and can perhaps fix it without pulling the head. Odds are however, it is a trip to the head shop for work.

Good luck,

Leonard