If you are just beginning to teach your horse to turn around, then you can't really do it by just pushing them around from a standstill. You need to have them moving forward.
To start, get your horse going in a circle at a walk with his nose tipped slightly to the inside. Bump him gently with your legs to encourage him to keep moving forward, and bump him slightly with the bit on the inside to keep him tipped in. Your outside rein should be your stabilizer, and should stay still at this time. Make your circle smaller so that you are spiraling down smaller and smaller. When you have reached the center point of your spiral, you will need to "open the door" with your inside leg so that he has somewhere to go, lift your inside hand straight up (that you have been bumping to teach him to keep his nose in and to keep him soft in the face), pushing with your outside leg and bumping a bit with your foot, and bring the outside rein onto his neck to help push him over. Cluck a bit. You should feel him cross over and start to come over his hock.
When he gives you a few steps, push him forward and walk him out before going back into it.
If you feel him start to back up rather than coming around in his turn, push him forward.
Once he has the basics, you will be able to make your cueing less significant, and he will lighter and will understand the concept from a stand still.
When a horse is pretty broke at it, you can also do the spiraling from a jog to help them learn to spin a bit faster.