Well, the type of hub depends if you want a freewheel singlespeed which allows you to coast or a fixed-wheel singlespeed which doesn't. If you don't have much experience building bikes, this might be a bit of a big step, because while wheelbuilding isn't anything like as hard as some people will tell you, it's not really something I'd advise a novice to start with. I don't know what size wheels your bike has, but you may be able to get hold of a prebuild singlespeed wheel in the size you need.