That's a very big topic. Have you got some books to start you off (John Seymour, Sara Raven, Monty Don give sound practical advice) ?
You can plan not just for summer or winter crops, but also for spring and autumn. Rocket salad for instance, does not do very well in summer because it's a martyr to flea beetle and also gets too peppery in hot weather. But it's wonderful as a spring or automn crop, and mine do very well at this very moment under cold frame. To some extent, the same apply to many oriental greens (Pak Choi certainly does much better as an autumn crop, even in winter under cloche).