Hi,
its obvious you are really keen and understandably impatient to get fish now that you have the tanks, but half the fun of getting a new tank going is the planning. If you plan it well it will be a success, but if you dont it could be an expensive and disappointing disaster. So try and hold fire a bit and do a bit of research - its actually a lot of fun and you will learn loads.
55gall is about 200 Litres and this will give you huge scope to make a really pretty and well stocked tank, full of beautiful fish that all get along togther. But before you get any fish you will need to get the tank ready, which may take a couple of weeks. There is no point in buying fish and putting them in an unprepared tank, most of them will get sick and may die.
The first thing to do is to decide where the tank is going to go and have a really strong stand for it as 55gall will weigh about 200kilos once filled with water and the gravel/ornaments and the weight of the empty tank will increase the weight. So the stand has to be really strong.
It needs to be out of direct sunlight, near electric sockets and preferably not in a hallway or very busy area of the house. If its going to be upstairs your parents need to check that the floor joists are strong enough or it may end up in the room below!! Make sure you are happy with its position BEFORE you fill it with water, its gonna be impossible to move it in the future without emptying it first
The next thing to do is to test your tap water for pH ( how acid or alkaline it is) as every type of fish has its own requirements to stay healthy and you have to choose your fish depending on your water... at least to begin with. You can get test kits to find out the pH from the Local Fish Shop (LFS) or from your water authority. Once you know the pH you can then choose fish who will be happy in that water. Many fish are happy in a wide range, so dont dispair!!
If you haven't got them with the tank you will need a really good filter (Fluval are very good and reliable, a Fluval 4+ would be best I think) and a heater, probably a 300 watt one. Also an air pump so that you can have some airstones ( bubbles rising up from the bottom) or ornaments that "blow bubbles" .
I get most of my fishy stuff from ebay as I cant afford full price for these sorts of items.
So you have lots to think about, get the environment right FIRST and while you are doing that you can research the fish you fancy. Everyone here will help you loads if they can see you are trying to do it right.
The next thing will be to get the tank water ready for fish - called cycling, but I'll save that for another day, this post is long enough already.
Good luck