The previous answer sums up the aspects of buying a squash racket very well, but here are some additional thoughts.
What has to be recognized when buying a squash racket is that it's very personal, and it depends a lot on your strengths/weaknesses as a player, as well as your level. The problem with buying a racket as a beginner is that it's hard to recognize strengths, weaknesses and style, as you most likely haven't been "shaped" as a squash player yet.
Personally, I like to begin with looking at the weight of the racket. That is, I try to decide what overall weight I think suits me, and what balance I think I'll prefer.
Rackets with different weights and balances will have different characteristics. A common misperception is that light rackets are better, but that is simply not true. A light racket is most likely better for an advanced player, but is isn't necessarily better for a beginner. Light rackets are more maneuverable, but they require a good player to handle them, as they are not as "stable" as heavier rackets. Also, with a lighter racket, the player himself must generate all of the power in the swing.
A heavier racket, on the other hand, is quite suitable for a beginner. This is because it is easier to handle (despite being less maneuverable) and it helps a weaker player to generate more power in his swing.
The balance is also something to keep in mind, and it is rather easy to understand why.
A head heavy racket will help you generate more power, but offers you less control of the racket. This is generally good for "weak" players. To understand the principle, look at for example a hammer, that is extremely head heavy and helps you generate a lot of power.
A head light racket will do the opposite. It won't give you any extra power, but it'll make you feel in control of it, which is good for powerful players. Think of it as a fishing rod, which has a very low balance and is very easy to handle (but it won't help you fight of a crocodile, that's for sure...).
A racket with an even balance is a mix of the two, and the most common balance of squash rackets.
Personally, this is what I find most important to look at, but you will have to try different rackets out yourself. My advice is to buy a racket that you think will suit you well (based on what I've written above), and then ask people if you can have a go with their rackets at your local squash centre, simply to get to try different rackets and prepare yourself for the next purchase.
Also, don't get fooled by the price tags. The most expensive racket isn't necessarily the best one. Lighter rackets tend to be more expensive, but this isn't because they're better but because they require more expensive materials to produce them.
End note: I play a Harrow Vapor, which I'm very satisfied with. It's quite light, but with an even balance and it's still quite forgiving. I also own a Harrow Spark, but this one is a lot harder to play with (it's better than me!), and a Prince Sovereign TT, which is a great beginner's racket (and rather cheap).