Fish are great starter pets to get. Lets face it, they're not very demanding, they are relaxing to watch, and they make great kid's pets. But there are some things that you should keep in mind before starting out a new family aquarium.

What size aquarium should you get?
Ultimately this is really a question of how much money you want to invest in an aquarium. Generally, the bigger the aquarium, the more room for error you will, as chemicals produced by the fish or chemicals in the water you add will take longer to get to lethal concentrations. The other side of the this is large aquariums require bigger filtration systems, sturdier stands (1 gallon of water weighs 8.33lbs, so a 100 gallon tank is going to need something that can support 833lbs.) and they generally cost more. Know where you would like to place your aquarium and how much room you have for it.

Room Placement
Something that wouldn't seem too important but it can be. The biggest thing to make sure of is to keep it away from drafty places. If you have gold fish (carp) it's not as big of a deal, but tropical fish and salt water fish all should be kept somewhere where free of a draft. Keeping the tank by a window has been debated over how good of a thing it is. As long as it's not drafty it should be OK, just keep in mind your gonna have a problem with algae growth.

Start Small
People get excited about adding fish, why not, it's why you got the aquarium in the first place. But don't go crazy. The best thing you can do is set the aquarium up, drop in a few fish flakes and let the aquarium sit for a week or two with the filter running. Why? Once you add fish they are going to start living (and eating and excreting) in that tank. Unless the tank has some beneficial bacteria living in it the ammonia could become too much for the fish to handle and kill your kid's new pet "fishie." Another way to get the tank "cycled," or creating a colony of ammonia fighting bacteria, is getting a starter fish (or two, depending on your tank). These guys are hearty fish that can deal with high levels of ammonia and nitrite. I recommend platies or some tetras, as long as it is not a neon tetra. Neon tetras do not make good starter fish.

Know the Nitrogen Cycle
I've been alluding to this for a while now but I'm finally going to explain it. The nitrogen cycle has to do with the break down in waste. Fish create waste, as does uneaten food, which breaks down into ammonia, which, is poisonous to fish. However, so happens some bacteria lives on the stuff, and they in turn turn the ammonia into nitrite, which unfortunately, also poisonous to fish, but not as so as ammonia. Another set of bacteria then break down the nitrite into nitrate, which isn't poisonous to fish unless it is in high numbers. This is where algae starts to grown and turns the nitrate into nitrogen and oxygen. If you want to know when you can add more fish, watch this cycle. The ammonia levels will peak, then drop as the nitrite levels rise, which will also drop and lead to higher nitrate levels. Once this has happened (and you have performed a water change) you can add more fish. You can help out the bacteria by including an under gravel filter which will turn the filter bed into a bacteria breading ground. Also including pores rocks will create pockets for the bacteria to live in, and of course, your filter should include a bio filter for the bacteria to live in as well.

The one inch of fish rule
This is a common rule for fresh water fish. It states one inch of fish per gallon of water. It's actually more of a general guideline, as more aerated, better maintained aquariums will support more fish, but it is a general target, and good idea to stick to. Keep in mind the inch per fish should be the fish's maximum size, not it's current size. If your not sure what it will be, ask your pet store. Should you happen to own a salt water tank (which if this is your first tank I highly advise against) you should stick with 2 inch of fish per gallon of water.

These basic rules should help you in setting up your first aquarium. Good luck!