I started with a 28 gallon and had a lot of success, but I did a TON of research before starting. Now I have an 8 gallon and it's doing really well, but I am itching to get into something bigger because my options are so limited. I think a good choice for a beginner is one of the larger all-in-ones (24-30 gallons). The Oceanic BioCube is really nice, Red Sea Max, etc... It really depends on how much you want to spend, but when you are just staring out it is nice to buy something that comes with just about everything you need. The less guess work, the better.
Good Luck!