Do you have a any experience? Did you use a sewing machine before or will it be the first one?
First off, you should check out some website for classified ads like http://www.for-sale.co.uk/sewing-machine
and do some search online. Then, of course, it depends on your budget. Would you like to buy a new or a used one for sale. The latter are usually cheaper.
Lastly it also depends on what you want to do and which materials you are going to use. If you provide some more information, I can give you some advice.