Every racket is different but here are a few general guidelines to help you choose.
The bigger the racket head the more power will be generated and larger sweet spot.
A lighter racket can be more quickly manoeuvred and allows quick reactions to volley.
A heavier tennis racket generates more power and has a larger sweet spot. The downside is that the extra weight takes more effort to move and may make getting the ball out of the corners more difficult.
Lower string tension generates more power.
Higher tension gives more control.
Thinner strings generate more power and touch.Thicker strings last longer but gives less touch.
Beginner - Intermediate - Advanced
Here are 3 recommended rackets:
Beginner racket Prince tt Prolite (less than £35)
Improver's racket Tecnifibre carboflex 150 (less than £75)
Advanced racket Head flexpoint 160 (less than £80)