The range to start will depend on what you are making. In general, probably between 1.075 and 1.100 to start. Again, depending on what you are making, may dictate how often you check it. In general, I check mine every couple of days, when fermenting.
If you go to the sticky at the top of this page there is a thread that says "How to use a hydrometer for beginners" good place to start. If you run into any questions or problems, come back and ask us. Somebody will surely jump in and try to help. Arne.