In addition to what Xulfiqar has said above ....
You can't do it alone. Have someone put pressure on the rear seat or rear support carrier thingy. Once you press the bike downwards into the ground, you will have enough clearance to slide in a Ring Spanner, to open the nut behind exhaust.
Also, i can't quite remember, if it was 20, 22 or 24mm for rear Nut. Check the CG125 Guide (in my thread, by r1) for more info.
Removing the rear tyre is quite easy. You didn't have to remove the exhaust. If you have, its better to buy a brand new Gasket Exhaust Pipe (otherwise you will risk leaking the exhaust) ... this thing:
Anyway, let me know if you need anything.