This is getting repetitious (or,as I prefer to put it, great minds think alike):
1) Don't buy a tank smaller than 10 gallons if you intend to keep anything more than a lonely betta.
(15-gallons tanks are fine if you have enough selfcontrol not to overstock and stick to small fish)
2) Cycle the tank. Fishless is best. Don't believe the people who say fishless is harder; its' not that hard. Cycling with fish is a lot harder on the fish- and that is what should matter. YOU don't have to swim in a bath of toxins. And you're going to have to learn about the chemistry of the tank anyway.
3) Buy a test kit, preferably a liquid one.
4 Never EVER buy a fish without researching it. Leave your money at home the first time. And do not expect that "for your sake a miracle be wrought"! Schooling fish will need to school; aggressive fish may seem perfecty sweet in the shop tank, and for the first few days/weeks/months at home, but one day pandemonium will break loose; gender ratios need to be observed etc.
5) Make time for weekly water changes (20-25 %) and do NOT overfeed.
Can I add a number six:
6) Fishkeeping is just that: fishKEEPING. It is not retail therapy, but looking after real living beings. From time to time we see posters who are spending great sums buying more and more fish, yet seem strangely short of cash when it comes to buying a test kit. All fish, whether cheap or expensive, deserve to have suitable living accommodation and proper care. A happy and well-kept guppy is always going to be more impressive than an unhappy zebra plec.