Since I didn't have time for a tutorial:

You can cycle through your items with right click

and through your tools with you scroll wheel

If you have your empty hand selected (mushroom crosshair) you can sow mushrooms with "e" and harvest with your sichel using left click

You can grind snails and mushrooms at the mill to turn them into meat and flour which you can cook into soup and bread which can be sold at the truck

to buy/sell items hold "C" at the truck with your selected item (Mushrooms/snails cannot be bought/sold)

You can use the sower on your tractor with "1" (toggable) and the harvester "2" (also toggable) 

Acid: removes all snails for 60seconds

Fertilizer: When active increase mushroom harvest yield (stacks with rain)