Do your research: location, condition and general sale prices. Zoopla is a good place to start as you can check property prices and how much they have sold for in the past - see similar.
Make sure that you have a mortgage or funds in place. Remember to have 10% to hand for when the hammer goes down - this is to exchange contracts and you are then legally bound.
The key is in the buying. Consider your liabilites: revamp costs, solicitor fees, stamp duty and factor these into your top bid. Don't be tempted to go over your bid... let it go!
You need to look at the property. Research some basic structural information. For example, a crack on the outside wall in a zig-zag through the mortar is an indication of subsidance. Otherwise, get a survey done.
Take a pair of binoculars with you so as you can look at the roof line from the ground, any missing?
Check out enviromental factors... for example flood plain.
Keep your nerve!