Summarize:

Rainbow sharks are not particularly large, only about 6 inches (15 cm). However, they are semi-aggressive and very territorial. These fish need plenty of space and hiding spots to prevent confrontations.  You might be required to assemble the aquarium. Follow the instructions on the box, because these will vary from tank to tank. Purchase a heater and filter to go with the tank. Be sure that the filter is designed for a tank of your size. Refrain from using an under-gravel filter, because it is not compatible with sand, which is the optimal substrate for rainbow sharks.   You should also purchase a tank stand designed to accommodate an aquarium of your size. While it might be attractive to use a table or desk that you already own, these often will not be strong enough to hold a heavy tank. Similarly, putting an aquarium on the floor is very dangerous. The tank should be placed far away from environmental factors that can cause large variations in temperature: windows, air conditioners, radiators, and air vents. It should also be located away from noise and foot traffic. Avoid spots near doors, on the floor, or in a busy hallway. You should also try to find a place that is near a power plug, a water source, and where you have space to both work on and observe the tank. Gravel with sharp edges can scratch up and damage the rainbow shark. Very fine gravel or, even better, sand should be used as a substrate. Because the rainbow shark is territorial, you will also need plenty of places for the rainbow shark to hide.  Caves, tubes, and other tank decorations sold at pet stores can help prevent conflict between territorial fish.  You should also purchase live plants for the tank. These are a good source of distraction for fish and help to keep the tank clean. Tap water should be fine for your shark, though you will need to treat it. Fill the tank near to the top, but leave a few inches of headspace. It is necessary to keep a layer of oxygen at the top of the tank. Once you introduce the shark, you'll need to keep the tank covered with a lid to prevent the shark from jumping out. Tap water usually contains chlorine and other substances that are harmful to your fish. You can purchase a water testing kit at the pet store to determine what chemicals you'll need to make the water safe for your shark. You will probably need a sodium thiosulfate solution to treat chlorine and Amquel to treat chloramine.  To get more information about the chemicals in your water supply, ask the local pet store owner or call the water utility directly.  In addition to adding sodium thiosulfate and Amquel to the tank, treat the tank for pH levels. There are a variety of acids and bases that can be purchased at the pet store to adjust the pH level in your tank. Test the level and adjust the pH until it is in the range of 6.5 to 7.5 pH. It is important to allow time for the water to cycle and for the chemical composition of the water to stabilize. Continue to closely monitor the chemicals to verify that they are at safe levels. Rainbow sharks like a water temperature from 72-79 Fahrenheit. To keep the tank at this temperature, purchase a heater to go with your tank. Set the heater somewhere between the 72-29 degree temperature range. You should also be sure to keep your aquarium away from anything that could cause it to suddenly change temperature. For example, keep it away from windows, heaters, and vents. Even a shark cannot survive more than a couple of hours in the bag that the pet store gives you to bring it home in. When you bring the fish home, float the bag in the aquarium water for half an hour so that fish can adjust to the aquarium's water temperature. You can add some water from your tank to the bag to help the fish get used to the new water. Then use a net to transfer the fish into the tank. Discard the old water instead of adding it to the tank, since the waste in the water makes for an inhospitable environment.
Purchase  an aquarium that can hold at least 50 gallons. Find a safe location for your aquarium. Put sand and hiding places on the bottom of the aquarium. Pour water into the tank. Treat the water. Allow the water to sit for two weeks before introducing fish. Keep the tank at a moderate temperature. Move your shark to its tank soon after purchase it.