Abby, salt water is easy but the bigger the tank the better, it becomes it's own self-maintaining ecosystem. What gets hard is coral, and it is a lot more expensive.
I'm really more partial to freshwater planted tanks than salt water, but then you have to prune the plants regularly - it's essentially an underwater garden.
I personally wouldn't get the guppies - go for tetras or other tropicals. Avoid anything that live births
I have a 10g and had a pair of guppies and they were spawning 20-40 fry every month and it just got ridiculous - the other fish would eat the fry, but it got so bad I lost a gourami to over eating them and so the guppies, and their babies, went to the pet shop lol.
I like gouramis, I had iffy luck with the dwarfs, but I have a pair of paradise blue gouramis who have done wonderfully. I had them, a mystery snail, 2 corey cats, and a betta in my 10g. My 10g got broken down as many of those fish died (they were all between 2-4 years old, so they did great) and now I just have the 5g with one of the gouramis, the betta, and the snails.
I highly recommend mystery snails - they do a great job of eating any algae growing on decorations, plants, glass, etc. They will also eat any dropped or missed food, and you can have more than one. They come in some pretty colors too - I have a pearl white and a dark blue, almost black. I had a golden inca, but he was evil and ate 4 glowfish and a gourami
Apparently, if starved in pet shop tanks, they will learn to hunt. Moral of the story? Never buy the last lonely snail no matter how bad you feel for it
I have always been partial to long, not deep, tanks. Cleaning is easier, because you can reach the bottom, and you can do more with decorations. I like natural, planted tanks, but you can do so much with it. Decide if you want gravel or sand - gravel is a little easier to clean, but sand is best to plant in and is better on fish, especially if you get a fish that likes to bury under the substrate.
One of my favorite fish, which I had for many years, is spiny eels. They are small, they have adorable hook noses, are pretty easy to keep (you have to make sure they are eating - most will eat flakes, you can supplement with dried shrimp) and are a fun addition. I had a standard and a golden. http://www.aquariumfish.net/images_0...01b_medium.jpg
Hmm... that's all I can think of right now! Oh! One last thing - spend the money on a good filter, it will be your best investment and best friend