Don’t write too much, as it is a poster, not a leaflet. A picture might help to catch the eye, especially as yours could be chosen for display.
Write about any animal that lives in Britain. Your information must highlight its habitat (what sort of place it lives in, hunts in;

Many people will already have books with useful information. I have found a few websites but you may well find others: BBC (or this one for younger children), Hedgehogs, badgers (advanced), a slideshow (if you have Powerpoint), General links.
Before handing it in please check: the title is correct, the work is named on the back, the animal is a wild one, and it lives in Britain.