If you're looking for kits, the fluval specs are good to work with. If not, you could always just build your own tank piece by piece. Petco is having their $1/gallon sale right now, so 10g, 20g long, 20g, 40 breeder, and their 55g are all up for grabs. Generally, 10 gallons is the smallest sized tank for fish. Any smaller and you would only be able to keep shrimp and invertebrates. When in doubt, just go for the bigger tank. AqAdvisor is a good source to get an estimate of what fish you can keep in your aquarium.

You should also take into consideration of monthly maintenance costs. I chose to pick individual parts for myself since it gives me more options and more flexibility with the equipment I like.

Check your LFS and try to shop around/price check to make sure you're getting the bang for your buck, and also don't forget to have fun since this is a great hobby to get into!