This is specifically is a beta fish tank, but you can use different water conditioners and food for other fish.
Step 1: Wash the House
Begin to wash you decorations. For any fish you need to have for them a house, or a place to hide. A blank tank will make an unhappy fish. Wash the house with light dish soap to get rid of any harmful germs. Make sure you wash all the soap off. Dish soap, if not properly washed off, can be just as harmful as and dangerous bacteria to a fish. So make sure its all washed off of the house. Lay the decorations on a clean dish towel.
Step 2: Wash the Rocks
I would recommend using large river rocks instead of pebbles. They are much easier to clean. Get out a large bowl, and fill with lukewarm water, rocks, and a splash of light dish soap. Stir them around and let them soak a little. Rinse them one by one, then lay them on the towel.
Step 3: Wash Any Other Decorations
I like to have at least two things in the tank, this way your fish can be happy. Since my tank doesn't have too much floor area, I chose to have a "Floating Beta Log" as my second decoration. My beta loves this. When betas are happy they make "Bubble nests" They take air from the top of the tank with their mouth, and go to their certain spot, go underwater, then release the air. This makes little clusters of bubble at the top of the tank. My beta loves to use his log for this and often times "lays" in the log and just chills.
Step 4: Wash the Tank
Wash the tank with lukewarm water and a light dish soap. When you are done with the tank put it on the towel to dry.
Step 5: Fill the Tank With Water
Fill the tank with water (room temperature or as close as you can get to room temperature) Take out about two cups of water so when you are placing the items in the water it doesn't spill everywhere.
Step 6: Place All Your Decorations in the Fish Tank
Place the rocks into the bottom of the tank. Try to make a flat surface with them. Next put in the house. After you have put in the rocks and house, put in the other decorations you may have. I wont be using a filter in this tank because it is relatively small, and I like to wash my fish tank at least once a week. If you want to use a filter, make sure to follow the instructions that come with the tank or the filter you buy.
Step 7: Condition the Water
Follow the instructions for your water conditioner. All vary. Make sure that you let the water sit for 15-20 mins, 20 is ideal. This lets the water get to room temperature, and lets the conditioner soak in.
Step 8: Your Done!!!
Enjoy your fish tank! Be sure you put it in a place that gets natural light. Also make sure that any other pets won't be able to get it. Experiment with different designs of tank and decoration to your liking. Have fun with your new pet!