Get your tank running, check its not leaking. Get the filter and heaters going. When it gets up to the correct heat (use a thermometer) then you can add some bacteria (you can get it from your local aquatic shop).
Keep the tank running, and keep testing the water for ammonia, nitrites, and nitrates. When they are all at the correct level then you can add a few fish. Then leave it a few days, test the water again and if its ok add some more.
Dont do what i done when i had my first fish tank (a few years ago), i put too many fish in, to early, and ended up with a tank full of ill, and dying fish.
Ammonia and nitrites are lethal to fish, and it is so important that you dont put fish in until the levels are reduced to zero.
Hope that makes sense, and i hope it helps!
Oooh, i almost forgot!! DONT overfeed the fish!!!
Thats another mistake i made with my first tank. I ended up with loads of uneated food in the tank which rotted, and made the ammonia and nitrites shoot up.
Silly me!!
Fortunatly ive learnt my lesson now. Im now on my third tank, but this one is bigger then the others i have had before, and ive got different types of fish in it.