First you have to decided what you want the microphone for - what do you want to record - what kind of material do you want to 'mic' up.
Microphones for vocals are different to microphones used to record drums. There are omni directional microphones and uni directional microphones, cardoid and ribbon microphones. A decent microphone will set you back about 100 pounds sterling - a studio quality mic kicks in at around 300.
You need to do a bit of research and decide how much money you want to spend. I would suggest you pay a visit to 'Sound on Sound' magazine's web site (google it) and have a look around - they've got loads of articles (tutorials etc) and info on all kinds of studio/audio gear.