I can see that your new to the hobby, and real anxious to get everything going. Here is my advice to new people in the hobby, presented again, by popular demand.
DaveK's Standard Lecture #1 – Advice for people new to the hobby
The very first thing you want to do, before you spend any money on equipment or livestock, is get yourself a few good books on state of the art reef systems. Then read and study them, so you have some idea about what your are doing.
Here are two to start with -
The Conscientious Marine Aquarist by Robert Fenner
The New Marine Aquarium by Michael Paleta <---This book has an especially good section on fish suited to someone starting off in the hobby.
This is information that you can not easily obtain from the net. While it's out there, it's all over the place, and there is a massive volume of information.
Once you get that done, plan or rethink your system. What do you want to keep? Do you have the necessary equipment? Do you have the knowledge to keep the livestock you want? Do you have the time to dedicate to keeping a system and it's livestock?
Many salt water fish, corals and inverts often have very specific requirements. Some are extremely difficult to keep alive, even if you do everything right. Before you get anything, research it, and be sure it will work out in your system.
Lastly, you will find that most LFS people are clueless when it comes to SW systems. Verify their advice, until they prove otherwise.
Additional thoughts.
Saltwater systems, no mater what you do are going to be expensive. It is true that you can save a lot getting used equipment and bargain livestock, but many things are needed and they can cost a lot.
For buying new equipment you can figure spending about $35 to $50 per gallon of water in the main tank by the time it's all set up. So that 20 gal tank would cost between $700 and $1000 by the time your done. Using used equipment can lover this quite a bit, but it's still a lot.
Please make sure you can afford the hobby, not only the initial investment, but the ongoing costs, like salt, light bulbs, filter media, replacement parts and so on.