First choose a subject. Ideally something that interests you, you are going to spend a long time researching it/writing it, believe me. If sport as such is not floating your boat these days, think of other areas you are interested in, and see if you can relate that back to sport, eg psychology, politics - history, that sort of thing. Sports and obesity - selling off playing fields to fund school building, there's probably tons of things.
It is good that you are thinking about it early, as this gives you chance to check out a few leads, before deciding what to do. Ideally you want to pick a topic on which you can find alot of material, but not one that has been done to death.
To be more worthwhile it should be more than a piece of decription, and a little investigation/original research always goes down well - although that is more usual in post grad theses.
When you've decided and got it approved - stick to it like glue . When I picked mine , I kept coming across all sorts of other subjects which sounded far more interesting, than the one I had picked (Architectural salvage, since you ask). Resist the temptation to change horses in mid stream, as that can spell disaster - and use your time wisely to pick a subject that will sustain your interest.