Few suggestions...buy as large a tank as you can afford...much more stable environment..and i'd also suggest getting the longest tank you can which is better for oxygen exchange...and allows for more of a footprint which provides more cover than a tall tank for fish to set up territories...
Also, do lots of research on different equipment and definitely allow for things like filters, heaters, substrate, etc in your budget and take into consideration a bigger tank requires more expensive equipment. Things get expensive fast...and thats without fish in the picture yet.
RESEARCH! RESEARCH! RESEARCH!
I decided about 6 months ago that I wanted an aquarium....i've been researching ever since and still haven't bought one yet. You want to do things right the first time so definitely take your time. Also, you will probably change your mind 100 times about what you want in your tank..I know I have. Use forums like AA and don't be afraid to ask basic questions, everyone here is very helpful. Good luck and hope to read of your future successes.