Test for leaks first.
then get the gravel, but that in the tank, make sure it is washed first. Sand is also good as well, but that needs more washing.
then get decorations, wash them as well.
fill the tank up 1/2 way with water, put everything in place the way you like it.
Then fill up the other 1/2.
next, set up the heater and filter in the areas that they are supposed to go, you will see where the pop outs are on the hood. That is the location I usually put mine in.
now after all that is done, you need to cycle the tank, there are articalks about that in other places.
When the cycle is all complete, you know this by testing the water with a test kit avaliable at lfs or pet store chains, I usually adjust the water to the area I want it for the fish, hardness, PH and all that good stuff.
then you add fish that will fit in the tank and that are compatable to eachother, add them a few at a time though.
that is the basics of setting up a tank.