Well, obviously you need the Seasons EP.
Go into build mode and into the plant section. In there, theres a special button just for growing crops - click on the brown.. 'blocks of sand' and drag it out to the size that you want. If you want, add sprinklers, scarecrows etc.
In live mode, select a sim and instruct them to fertlize the land, or plant a crop. smile
Remember to water them and pull out weeds, and spray bugs away. whee