From boards spread around the whole map, you can learn what missions are currently available. To check it, you just need to enter the icon of an exclamation mark located in front of any job board. There are three main types of missions: mowing, transporting pallets and crop delivery. Each one requires appropriate equipment (a mower, a tractor with a front loader or a tractor with a trailer + supplies in the warehouse).
For completing a mission, you will receive a reward and for doing it fast - a bonus. There's no penalty for not completing a mission or abandoning it. You have about 10 minutes for every mission, so it's a good idea to focus on one at a time, to obtain the bonus for doing the task fast. Places connected to missions (for example a field that has to be mowed) are marked on the minimap with a green circle.
Remember that you can change how often new missions appear in the settings. This way, you will be able to decline those more difficult or impossible to complete at the moment and new ones will appear very quickly (once every 2 minutes).
The rewards given here are only approximate numbers. The bonus is also an average number, but the main rule here is "the faster, the better".