I will assume you want a reef tank, since your in the reef forum. If you are trying to get away with a tank on a budget, that is very hard to accomplish.
The main thing you absolutely need is patience.... This can't be overstated. If you rush you will not only waste money but kill or hurt animals. I highly suggest you get together a plan and map out what the next year you want to accomplish. This will also help come up with what everything might cost. But realistically you can add hundreds to that figure ask anyone.
You will need for starters. A REEF test kit, refractometer, "strong" lights depending on what you want to keep, a heater, Filters, thermometer, Timers for your lights, power strips, a source to get reverse osmosis or RODI water (very important) Some salt mix, mixing buckets, glass cleaners for algea The list will keep growing because I really haven't even scratched the surface but I don't want to keep going. GOOD LUCK Just keep reading the forums