1. Determine what kind of fish you want to keep.
Different kinds of fish will require different care, different conditions, different space, and different equipment.

2. Research care and compatibility of those fish.
This should involve your first purchase in the aquarium hobby - a book. It is very important to learn about the fish you want to keep. This will let you know what conditions they will need, what equipment you will want, and how to set up your fish tank.

3. Determine space.
Now that you know a little more about the fish you wish to keep, you know how large of a fish tank you will need. Look through your home and select a location for your new aquarium.

4. Determine budget.
How much can you afford to spend on your aquarium? If this is your first fish tank, how much can you afford to spend on a new hobby that you are not sure you will be pursuing long term?

5. Select equipment.
Visit your local pet store and determine what equipment is available and how much it will cost. If you do not see something that you will need for your system, ask about placing a special order.

6. Evaluate budget/space constraints.
How does your budget compare to the cost of the equipment you will need? Can you get a fish tank large enough for your fish that will fit in the space available in your home? You should address both of these questions, and then evaluate your fish selection, your space constraints and your budget compared to the new information you now have.

7. Purchase equipment.
Once you have reevaluated your fish selections, space limitations, and budget, it is time to purchase equipment. If you need to special order anything, do so early, as it may take a considerable time to get equipment by special order.

8. Setup equipment.
Now that everything is home, clean it all off and get ready to go. Expect to spend a couple of hours setting everything up if this is your first fish tank. Fill your fish tank with water once all of your equipment is set up, and let it settle for a day or so so you can make sure that everything is working properly and that nothing leaks.

9. Select starting fish.
During the first day ir so, while the fish tank is running without fish, you can go back to your list of fish and select a few starter fish. These fish should be hearty, inexpensive, relatively small, and something you want to keep in your fish tank in the long run.

10. Cycle your fish tank.
Over the next 6-8 weeks, you must be patient. Be very diligent with fish tank maintenance, be absolutely sure not to over feed, watch your fish's behavior closely, do extra water changes as necessary, and DON'T ADD ANY MORE FISH. Until your fish tank has finished cycling, you should only stick with your few select starter fish.

11. Maintain your fish tank.
Once the fish tank has finished cycling, feed and observe your fish daily. Check your filters at least twice a week. Perform a 10-15% water change every week, and scrub for algae at the same time. Every month, check all hoses, fittings, clamps, cords, lights and other miscellaneous equipment.
Source: firsttankguide.net