A box call is nice because it is really loud and can almost be used like a locater call. You can step out in the woods and use the box call in a loud fashion and try and get a gobbler to sound off. Once he does, you now have an idea of which way to start walking. A slate call can be used in the same fashion, however it is easier to use a slate call quietly (if needed). A mouth call is pretty much the most versitile call. It can be used quitely or more loudly and it doesn't require the use of your hands. Since you don't have to use your hands on a mouth call, your hands can then be either on your gun and ready for the shot, or you can even grab another call (such as your slate) and start to mimic two hens grouped together. I would strongly advise getting a mouth call and at least practicing with it enough to be able to do a really nice yelp, which is the most basic of all calls.

I would go ahead and get a locator call and try it out. I have a friend that goes through locator calls like there is no tomorrow. He gets gobbles to a locator call and then starts walking towards them and as soon as he feels he is close enough, he switches to a mouth call and starts working the bird in. He is probably the best turkey hunter that I personally know, and I have never heard anyone use a mouth call as well as he can. I haven't had much luck with locator calls where I hunt, however, my friend who lives several thousand miles away from me swears by them. It is something that you are going to have to try out for yourself and learn firsthand. You can read all the books and watch all the videos possible and know how to make all the calls perfectly, but the only real way you are going to learn to turkey hunt is to go out and expierence it for yourself. You will make several mistakes and will become a more confident turkey hunter over the years.