Hi. Welcome to the forum. I'll add a few comments to help steer you in the right direction, but there's no way i can type out everything you need to know. This site is a great source of information, so the best thing to do is to read as much as you can. I think you're friend has probably done you a favour steering you away from a marine tank for now. Get your feet wet with a freshwater set-up to see how you like it. I can't really tell you prices as i don't know where you live, but you can figure that out easily by looking yourself.
Ok, so if you want to do a tropical freshwater set-up you need 1. A tank. 2. A stand to put the tank on. 3. A filter. 4. A heater. 5. Some sort of lid. 6. Lights. 7. Decorations 8. Fish. 9. All the little things. 10. Knowledge.
1. The tank. The general rule is to buy the biggest tank you can afford. Bigger tanks are more forgiving as there is a greater volume of water, plus you can have more, and a larger selection of fish. You'll have to choose between glass and acrylic (which really doesn't matter although each has it's positives and negatives). You may also be able to get a good deal on a used tank.
2. The stand. You need a really sturdy stand to put the tank on that can support the weight of the tank, water, and all the decorations safely that has a level surface. You can buy new/ used, or build your own if you have the skills and tools. Something that has some storage space will be a help too. Remember that a gallon of water weighs more than 8 pounds so a flimsy table isn't going to cut it.
3. As for the filter, there are different types. For an aquarium up to 55 gallons you might consider a filter that hangs on the back of the aquarium. Aquaclear, Penguin, Emperor, Tetra etc. are all common brands. For larger aquariums, or if you prefer, there are also canister filters which are often housed inside the stand. The goal with any filter is to get one that can turn over (ie filter) all the water in the aquarium at least 4-5 times per hour. So a 20 gallon tank should have a filter with a rating of at least 80-100 gallons per hour flow. More is always better, you just have to make sure that you don't get a huge current going that will bother the fish. Lastly, don't get an undergravel filter... read up on some posts about them and you'll soon discover why.
4. The heater. There are two basic types: hang-on or submersible. These are pretty self explanatory, with the submersible ones you can totally immerse them in water. The hang ons hang on the back of the tank. There are also inline heaters, but unless you have a large, fancy set-up you won't need one of these. The rule here is that your heater wattage should be rated at 3-4 times the size of the tank. So a 20 gallon tank would need a heater of 60-80 watts minimum. I prefer a little extra wattage which helps in the winter months. I also prefer the submersible type because you can hide them better, they give you more flexibility where you can put them, and you don't have to worry about it getting totally wet.
5. The lid. Some tanks come with plastic or glass lids, or may even have a wooden lid or "hood." The lid serves a couple purposes. The main thing is that it keeps the fish from jumping out, and it gives you a place to hide or place the lights. If your tank doesn't come with one, make sure you buy or make something.
6. The lights. If you're not going to grow live plants, any old lights will do. All they need to do is allow you to see the fish. Often the lights are incorporated into the lid or hood.
7. Decorations. You can put gravel, or sand on the bottom of the tank or leave it bare. Then decorate with real or fake decorations. But beware, some real decorations will alter the water chemistry. Some rocks will raise the pH, some wood will lower it. So research and be careful before you add any natural items into the tank. Also, metal is a no-no.
8. The fish. Research the fish you want to buy; look in the store etc. There are so many kinds that i couldn't begin to offer much advice here. It all comes down to personal preference. When choosing a fish you should make sure that it won't grow too big for your tank (no they won't only grow to the size of the tank), you should try to make sure that your water is acceptable for the type of fish you're buying. Many types of fish prefer to be kept in schools of 4-6 minimum so that is a consideration when buying. Lastly, don't buy too many fish and overstock the tank. You'll run into problems and will likely lose some fish. The general rule is 1" of fish per US gallon of water. The catch is that the rule applies to adult sized fish, not the little ones in the store, and you don't include the tail in the measurement. So in a 20 gallon tank you could have say 20 one inch long fish, or 10 two inch long fish etc. You can bend the rules here a little under certain circumstances, but it's best to stay within the guidelines at first. You want to make sure your tank is at least 6 times longer than your longest fish will get, and at least double as wide.
9. All the little things. Food, buckets, gravel vacuum (siphon) for cleaning the gunk from the bottom of the tank, a net, dechlorinator, etc. etc.
10. Knowledge. Make sure to read up on the nitrogen cycle (on this website, or even google it), and also the fishless cycle. The more research and reading you do before getting started the less trouble you'll have, and it'll save you some money in the long run.
I hope this will help you out some by giving you some good advice before you start to help you be successful from the beginning and maybe save a few bucks too! It really is a great hobby, and once you're set-up it isn't that expensive either. So good luck, and happy reading!
*edited for spelling.
