Buy the biggest aquarium you can afford. Reason; It's a lot eaiser to take care of. Really. The water paramiters you will be after won't be as easily affected in a larger tank. If you over feed, under clean or forget to turn the heater back on after a water change,etc...these wont affect the overall condition of the tank as dramatically as they would in a smallish tank. I am a firm believer in buying good used equipment first and see how it goes...a good used 50-60gal tank. Glass wont scratch like acrylic, acrylic is stronger and lighter than glass. A hang-on-back type filter (or two) will give you the easiest maintenance regimen, but try and get one that will give decent bio-capacity, like one with a bio-wheel or bio-sponge or other such media. A sturdy stand is essential to keep the tank on and make sure the tank has a lid.Buy a NEW heater, make sure you have an understanding of the requirements of the fish you buy as far a diet and temp. Fish that you get that will require a specific anything can tend to be frustrating to new aquarist.
And finally, dont take anyone's word for gospel...we all have our own way of doing and you'll be able to, in time, figure out what works for you. Use this sight, don't be affraid to Google something and don't forget the library. Arm yourself with knowledge and this hobby can be very fulfilling!
WELCOME TO FISH KEEPING! WELCOME TO MFK!