Allotment virgin!

Hi Gillyr and welcome to the forums 

You don't say how much clearance may be needed, so probably the best place to start is to have a look at the articles about clearing and planning a plot here

You can discard the bits which aren't relevant.

Also on that page are a series of monthly articles indicating what you should do at that time.

Don't know much about green manures, you could try a search of the forum if noone else responds to this point.

But the most important thing right now is to decide what you want to plant and where on the section you hope to get - some things e.g. garlic and spring cabbages will be going in soon-ish so you don't want to put green manures there.

P.S. I'll move your post over to Grow Your Own for more people to see it