If it is with 123reg log into your control panel and go to the web forward but, type in the address you want it to redirect to, then go to trouble shooting and follow the intrusions. They give easy to follow instructions, if you follow it exactly it's easy, if you try and be cocky like me, it's hard lol
I have a feeling it's only that simple if you have your hosting with 123 reg too. If you just have a domain name, you have to change the name servers to point to your hosting provider.