Use slide film. Use print film. Yes, you can get prints made from slides, but it's expensive and the quality isn't as good as prints made from negatives. If you choose slide film, you will not get negatives. Remember for print film you have developed at YORK, you can also request your pictures on disk.
Film speed refers to how light sensitive it is.
The higher the number, the more light sensitive, and generally the more grainy. While you can purchase ISO 1600 film, you'll probably be disappointed with the results unless you're a very skilled photographer. For low light situations or to make your flash more effective, try ISO 1000 film. For all around shooting, indoors and outdoors, with flash and without, try ISO 400. If you're sure you'll be shooting a whole roll in bright sunlight, choose ISO 200 or even ISO 100 if you don't expect to be shooting moving objects.