Annabel,
Have a look at this but don't let it put you off!!
http://www.runnersworld.co.uk/forum/training/training-for-the-right-distance/166480.html
If your at uni or college they are likely to have a club? Or join a local club. I believe they cater for all abilities.
Probably best to target 5k as a starting point. And go from there.
Find a couch to 5k plan, follow that and see where you want to go from there.
Edited: 21/09/2012 at 16:49