Missions are found in the pause menu, they are also called "contracts".
Some tips:
1.) Avoid potatoes, beets, and sugar cane - harvesting and planting these crops is very, very time consuming.
2.) Fertilizing missions are very, very fast - particularly with the Hardi Rubicon 9000
3.) Plowing missions are generally pretty slow (narrow equipment)
4.) Cultivating, Sowing, and Harvesting missions can be somewhere in the middle
5.) Transport missions are very quick,
if you are good with fork tines to move pallets, or you hunt down an autoload transport.
6.) The mod named "better contacts" or "more missions", both of which are available on the in game modhub for PC allow you to run more than 1 contact at the same time - it can really help maximize efficiency.
Continue in this vein, and you will eventually unlock: