Choosing universities is a process that I think changes from person to person. You have to consider whereabout you would like to be living; whether or not you'll live in halls or commute from home and of course what subject it is you want to be studying. There are several different tools to help with this decision the main one being the league tables which are updated annually and also include the grade/UCAS point requirements for each university (and you can filter this search accordingly to your own interests to see which universities have the best results for your chosen subject).
The five universities that I chose to put on my application were Southampton, Brighton, Northampton, Lincoln and Exeter. I actually found it all lot easier to choose universities that I was interested than I was expecting. They all offered the course I was looking for, but it was actually the atmosphere/environment that won me over in each case. As I was moving away from home, I wanted to experience a completely different style of life and all five of my choices offered that. I think the most vital thing to remember is that whilst its nice to have ambitious goals, its important and in your best interests to be realistic otherwise you might end up disappointed with the outcome. Only you are aware of your own capabilities, so don't force yourself into applying for something you know you just won't be able to meet the requirements for.