There are many factors which need to consider when choosing running shoes:
1) level of arch - need to match your foot obviously (I am very flat footed)
2) the way your foot lands when you run. There are specific designs for over-pronators and under-pronators (I am a severe overpronater, so if I used wrong shoes there is a high chance that I will twist my ankle)
3) what surface you intend to run on
4) cushioning
5) more that I can't think of
The right shoe might not be cheap, but it will help prevent injury
I recommend the London Marathon Shop in Covent Garden. They assess the way you walk and can tell what type of running shoe you need. I was very happy with their service and will happily buy from them again (although they are not the cheapest place).