Because of the way loans are structured, in the first years you'll be paying mostly interest in your monthly payments. In the final years, you'll be paying mostly principal. If you have everything with you when you visit your lender, you'll save a good deal of time. Depending on your lender, you may be asked for other information. You're right - there are many types of mortgages, and the more you know about them before you start, the better.
Most people use a fixed-rate mortgage.
In a fixed rate mortgage, your interest rate stays the same for the term of the mortgage, which normally is 30 years. The advantage of a fixed-rate mortgage is that you always know exactly how much your mortgage payment will be, and you can plan for it. With this kind of mortgage, your interest rate and monthly payments usually start lower than a fixed rate mortgage. But your rate and payment can change either up or down, as often as once or twice a year. The adjustment is tied to a financial index, such as the US Treasury Securities index.