If you put any about of time into skateboarding, there is going to come a time when you want to pick a good skateboard, and work on some tricks. There are loads of different internet shops, and a whole world of choice. All you have to do it pick.
Break open the piggy bank. How much money do you have in there? Essentially, this limits your ability to purchase what you want.
Ask other skateboarders. At some time or another, all 'boarders have to take that final leap of faith, and splash out on a board. They have probably already tried out several boards, and can help you make the right choice. Also, they will know about bad buys.
Take a look on the internet. The best thing about websites is that you can actually see what you're buying. Obviously, you can't tell how it's going to feel under your boots, but you will also be able to look on skateboarder forums, and read what has been said about various boards.
Borrow a mate's skateboard. You've probably borrowed boards already: was there one that you were especially fond of? The only way to be sure that you're going to get on well with a board is to try it out.
You can go for named brands. These are more reliable, and you'll be able to find good parts (like trucks) easily. They will also cost more, so it is really a matter of economics.
If you are only a beginner, most available skateboards will do. Don't splash out on anything too flash, as you will probably find you damage it a lot.
If you are more interested in tricks, you will need to look for a board with a good pop. You will probably also need to find a second income to pay for the thing.