Question

how did humans start agriculture

Answer 1

here is the answer you want

Answer 2

There is a great chapter or two in the book Guns, Germs & Steel by Jared Diamond on this topic. Hunter & gatherers were finding what grew in the wild and over time figured out that many of those crops would propagate from seed and began harvesting those and later saving and growing more. Finding crops that could be stored for a long time was a true turning point as that allowed people to stop the migratory patterns associated with hunting and gathering. 

The domestication of wild crops and animals led to the creation of agriculture and thus the cradle of civilization, with more people being able to pursue things other than food. The Fertile Crescent in the Middle East, parts of ancient Asia and other parts of the world had people domesticate the crops that were already growing wild there. The earliest cultivated crops are thought to have been: 
Europe and Asia (wheat, barley and rice)Tropical Africa (sorghum and yams)Americas (corn)Papua New Guinea (taro)Many fruits come from perennial plants -- think apples, grapes, etc so they would come back year after year. I haven't seen speculation that fruits & veggies came first but more of the paleo type diet of proteins & grains