Here are a bunch of tips I have found for Tap Farm, the farming app for the iPhone that is taking the mobile gaming community by storm. This will give you a lot of tips but make sure and visit the resources link at the end of this post for more information.
Tap farm starts out with a few barren plots of land in which you are expected to plant crops. And it turns out that crops are the best initial and basic producer of experience, and that's what you'll likely play with first. Your first efforts were probably spent farming tomatoes every three hours until you could fill up your farm with 15×15 patches of crops. Then you probably started wondering which of these crops yields the most experience.