Firstly find out how big a mortgage you can afford.
See a mortgage advisor for more advice about this.
Secondly, once you know the amount you have to
spend, decide on the property that you want to
buy. The property search feature of this site
will help you.
Thirdly negotiate with the agent the actual purchase
price of your desired property.
Having agreed a price find a solicitor to arrange
all the conveyancing work.