Hello, welcome to JustAnswer! I am a licensed veterinarian, and I'll be happy to help you in any way I can.

Prior to adding fish, you need to cycle the tank properly.

This can be done by adding ammonia, to simulate the waste products the fish will produce.

this ammonia will eventually be broken down by bacteria that are critical for a healthy tank to function properly.

Here's a good link to walk you through the cycling process:

http://www.csupomona.edu/~jskoga/Aquariums/Ammonia.html

Once the tank is established, the level of maintenance needed will depend on how many fish you have, how many live plants you have, and other factors such as how much sunlight the tank receives, and how much you feed.

partial water changes are usually done a few times a month. I find that with tanks that have not too many fish, and a lot of live plants, the need for water changes is less.

levels of nitrate, nitrite, ammonia, and pH should be checked at least weekly once the system is stable.

the substrate (gravel) at the bottom will need to be vacuumed with a gravel vacuum monthly or so, to clean up solid wastes from the fish.