A few great options for cleaning tanks and materials:
Water with Aquarium salt or Seasalt (non iodized and no additives)
Vinegar is also non toxic to fish, just rinse well
Never use bleach, chlorine is deadly to fish and no matter how much you rinse filters and tanks you wont get it clean.
As for the gravel, put it in a bucket, add a hose and rinse till clear.
Remember, when you start a tank new, you will go through a cycle once you add fish. Many fish will NOT survive a cycle. A Cycle is the Nitrogen Cycle. When fish pee and poo, ammonia levels in the water rise. Very toxic to fish. A type of Bacteria starts to grow in your gravel and filter called Nitrobacter type 1. It uses up ammonia and transforms it into NitrItes (also very toxic to fish). Once the NitrItes reach a certain level and all the ammonia has been used up, a Second type of bacteria grows in gravel and filters called Nitrobacter type 2. This uses up the NitrItes, and converts them a relatively innocous and harmless type of chemical called NitrAtes. If you monitor your water with drip tests purchased, when the Ammonia and NitrItes have dropped to zero and the NitrAtes are risen, your tank is said to have "cycled"
Now it is safe for fish. You can fishless cycle in a few ways. You can add a peeled raw shrimp to tank and let it rot. You can add a few pinches of fish flake every day. Or the easiest way is to use a few drops of household ammonia daily. Any cycle you choose will take anywhere from between 12-60 days, so be patient.
If you chose to use fish, which I wouldnt recommend, use Zebra or Leopard Danios. They are incredibly hardy, tough fish that will withstand the cycle. Some may die, but not as much as other fish. I would add 6 Zebra Danios and let the tank cycle. Remove the danios once the cycle is complete, return them to the fish store for store credit and get what fish you want.
Then, if you want some help choosing fish, I can recommend some different types depending on the tank size!
Here is a SUPERB article on Cycling for you, it will get you started.
http://faq.thekrib.com/begin-cycling.html
And a great site with information of fish species, forums for fish help, and very knowledgeable people:
http://www.fishprofiles.com
