There are test kits that you can buy at walmart and target super stores. Also wherever you bought the fish at should have them as well. All you have to do is dip the test strip in the water and it will turn colors. The bottle will tell you which color means what and also what levels are acceptable. Ideal is 0 for ammonia and nitrites, and I think 40 or so is tops for the nitrates.
Basically when there is waste in the tank, from excess food or fish waste it decays and produces ammonia. Natural bacteria in your tank will grow and "eat" the ammonia, but will produce nitrites. Once the ammonia is broken down, a different bacteria will eat the nitrites, but produce nitrates. Nitrates are the safest of all of them so long as they are under a certain level. The nitrates will continue to build until you change the water. It is recommended that you change approximately 20% of water about once per month. Any more than that will get rid of too much of the good bacteria.
You will need a gravel vacuum to get rid of the debris in the gravel. They are around 10-20 bucks for the cheap ones (which work just fine) and upwards of $90 - $100 for the self syphoning ones. Vacuum out your tank once per month when you do water changes. AFter you've cleaned your gravel, take out additonal water until you've reached 20%. If you are vacuuming and find that it will take more than20% of the water, keep going until a max of 40% of the water is taken out. If you are still not done, then stop for now and repeat in a few days. When you put water back in the tank it is also a good idea to put something in there to take out the chlorine. You can use Stress Coat, Prime or the like. On the bottle it will say what it is supposed to get rid of. I highly recommend doing more research. There is a lot more to fish than just feeding them. Also when you clean the tanks, you will probably need an algae scraper to get rid fo the scum and algae on the glass. It looks like a green sponge with a handle on it.
Oh, btw, your sucker fish is most likely a Chinese algae eater or a plecostamus (sp?) also called a pleco.
Before getting ANY fish, do your research. People at these places are rarely knowledgeable. I'm sure the sales guy failed to tell you that guppies get sick really easily and also reproduce easily.