One part that the bike that doesn't get enough focus is the seat. Having the seat adjusted at the proper height is very important for your joint health and for your comfort while riding.
Fixing the seat at the right height is an easy thing to do. First, rotate the pedals so that one of them is at the bottom of it's stroke and the other is at the top. Place your foot on the lower pedal and bend your knee to about a 25 to 30 degree angle. Take note of how high or low you are and set the seat at that height. To set the seat, there is a clamp on the frame where the seat post enters the frame, just tighten that and you're done!
There are all sorts of reasons why you might need to change your bike seat height to accommodate different riding scenarios, especially in mountain biking or long distance road biking. This being the case, it is always helpful to mark the seat post at any of the heights that work best for you.
The reason to have a bike seat at the proper height is because if the seat is too high, you'll constantly be over-extending your knee. And if the seat it too low, you won't be able to get enough power into the pedal stroke.
If there is a bike shop near you, it might be educational to go down there and talk to the mechanic, or you could simply ask the internet some more questions about your bike seat and do your research that way.