Take the certificate of entitlement, your registration document plus your tax disc to your local DVLA and within a very short time you will be issued with a replacement disc with the private plate number on and then you will have to get some plates made, maybe at Halfords unless you get specials made up at one of the online companies. I think that these days you have to prove to Halfords (or whoever) that you actually own the car so be prepared!
Do you know where your nearest DVLA office is? If not, which is your nearest big town?