You should test your water on a regular basis.  Test strips are available that check acidity (pH), ammonia, nitrites and nitrate levels of the water.  Inexpensive kits are available in pet stores that not only test the water, but provide chemicals to add to your tank to give you the correct levels.  Be sure to read all of the instructions carefully before testing and adding to your water.

Every week or so you should remove about 10% of your water and follow by adding fresh water to your tank.  Syphons are available to clean the gravel in your tank.  These are inexpensive plastic tubes with cones on the end.  Simply fill the tube with water from your fish tank and place the cone end in the gravel.  Place the tube end in a bucket on the floor. The suction action will stir up your gravel, removing the waste and some water while leaving your gravel in the tank.

After cleaning, restore your water level.  It is important to add water that is the same temperature as your tank water.  Tap water is treated with chlorine, which is harmful to your fish.  Before you add the water, treat it with drops that instantly remove chlorine from the water.  Some good brands are Seachem Prime, Tetra Aquasafe NH/CL Formula, Jungle's ACE, Kordon's AmQuel and Kent's Professional Ammonia Detox.  Be sure your label specifically states that it neutralizes ammonia as it removes chlorine.  If it does not, you will be adding harmful ammonia to your water as you take away the chlorine.