I will try to run through this and answer some questions. The top head is called the batter head and the bottom is called the resonant head.
Typically you would want to use a resonant head that is matched (or designed for use with) the batter head. (Keep in mind that this is not set in stone, I know drummers that mix head manufacturers to get the sound they desire and that is ok too).
Typically you will want the batter head slightly tighter than the resonant head but that varies with head type and desired sound (or what you think sounds good).
If you are playing casually and your kit is inside of a temperature controlled room all the time, then you can get away with changing the batters once a year to year and a half, and the resonants every couple of years unless they become damaged in some way (dimpled or ripped). You might want to change them sooner than that if you notice the heads won't hold their tune.
Here is a link to the Drum Tuning Bible which will help you to understand heads and tuning much better:
Drum Tuning Bible