The ballot for FLM opens in August. Pick up a copy of Marathon News in sports shops at the time for an entry form. Then you find out if you're in or out in November (about 1 in 4 chance). If you're not successful, you can get a guaranteed charity place in exchange for a pledge to raise a minimum amount of money.
www.crunch.org.uk (charity runners clearing house) is a good place to start if you are after a charity place.
As far as training goes, it really depends where you're starting from. Getting the long runs in as early as you can will pay dividends, but you need to get a good base of fitness before you go long. There's no point killing yourself on a long run and then not being able to run until the next long run.
If you tell us more about your current training (weekly mileage, number of runs, type of runs) we'll be able to help more.