Well start with finding an area in your home that you want to put the tank, find the tank that best fits that space. While your doing that read, read, read!!! There are a lot of helpfull articles in that section of this forum. Do your research before you buy. Always double check what someone at a local store tells you. And make sure you leave room in your buget for a good skimmer. It is one of the lifelines of your tank.
There's lots more but that should get you started.
the most important rule to learn: Patience
Hopes this helps a little