First of all you should play Fullscreen. When you spawn behind you are the garden spots and infront the shop.
On the bottom of the screen you have your inventory slots. With the mousewhel or buttons 1,2,3,4,5,6 you can equip the seeds you bought from shop and Press E over the garden spots . Then you need to equip the watering can and press E again over the spot you already have a seed planted. If you need to fight for any reason equip the last item being the punches and left click to punch . Hope you have fun !