Training my horse. Help !?

I'm training my horse with a lead rope first because he's rusty. He hasn't been seriously ridden for almost a year. Anyway, when I get the halter and rope on him, he does fine walking and stopping and if he gets a little angry I kinda give him a slap with the end of the rope. I can get his attention fine but he kinda wonders off after 10 seconds. He's a nosey and curious horse. But today, I trained him for about a hlaf hour for the first time in forever it seems and he was bucking about 2 inches off the ground and running crazy wild. So I pulled the rope around the tree to get him to stop and then unhooked the rope and let him go then he was fine. How can I show him that I'm the boss and I'm in control. I've never been scared around horses wxcept when I saw the way he was running..