I can't do less that completely agree with oddz. But I would like to take it a step further. What are you gonna do when even that very simple query is giving up on you? Ask around here if someone can fix it?
Anyway if that's how you want it! It all depends what you would like. A left to right slider, a cross fade etc. Here are a few links:
It is of course up to you but I would like to strongly advise you to take some efforts to go through the instructions. That way It will be way easier to adapt such a slider too your own needs.