The delegates are chosen using different methods, depending upon the state. You should e-mail your state republican chairman, to find how they're chosen, in your state. In the majority of states, delegates run, in the primaries, to reprepresent a particular presidential candidate, at the convention. Those delegate candidates can be delegates only if the candidate they support wins enough votes in the primary. Some states have a "winner take all" system of convention delegates, and some states let delegates vote for different candidates.
Do you want the republican nominee to be chosen before the convention or during the convention? I hope that no candidate wins enough delegates before the convention. That would cause more suspense, and the convention would be more exciting, causing more people to watch the convention and hear the great republican ideas.