You can start now by digging over your plot and digging in some well rotted manure if you can get some . Riding stables are a good bet if there are any near you they have loads of it some of the veg you can grow is winter based such as winter cabbage brussel sprouts and leeks for example . Most vegetables are grown from seed though onions are mostly grown from 'sets' which are small onions . If there are any allotments near you they may have an allotment shop where you can buy discounted stuff for your garden and small plans to start off instead of growing your own seeds , but growing stuff from seed is dead easy as long as you follow the instructions , a good tip is to buy one of those Dr Hessayason books on gardening they are only cheap and give a lot of basic info