Read up on that link sik80 gave you. You'll need to buy a water test kit (to test for ammonia, nitrite, nitrate,and pH) and a gravel vac so you can change the water. You only change a portion of the water. In my 20 gallon, I change 30-50% of the water each week. However, while your tank is cycling, you will need to change it more frequently. ALWAYS use a water conditioner/dechlorinator when you put new water in your tank.
On the top of the webpage, there is a blue menu bar. The second one over says "Tropical Fish Profiles". Click on it, and go to "Cyprinids" under "Freshwater Fish" There, you'll find information about the sharks. Try to figure out what kind you have. Then you'll be able to get more information about their care requirements.
Finally, on your filter question, we need to know what kind of filter you have. Or if your filter came with an instruction manual, it should tell you when is a good time to change the filter media.
Good luck and welcome to the forum and to fish keeping