The first thing you need to do to organise a walk is to pick a route. You could use a favourite walk, get a route from a book or recycle a walk you have attended with another group. Alternatively, you can try making up your own route. The OS 1:25,000 maps are available from Streetmap and Bing maps. Details of permissive paths are available from Natural England/Defra
Points to consider when picking the route are
You should make sure your first few walks are not too long or complicated. Try to get the OS Explorer map (with an orange front) which covers your walk – you may need more than one. If you don't have the relevant map yourself you may be able to borrow it from another member of the group.
If possible, try to find a start point that is on public transport. You could also try organising a lift share meeting point.