You need seeds to plant in your plowed land. If you have them, just pick the seed up (left click it, so that it follows your cursor) then right click on the plowed ground.
If you don't have seeds, you'll either need to get them off someone else or grab the Foraging skill and level your Exploration up to 8 or so. Then you can start finding Windsown Weeds. It's a good idea to keep levelling your exploration up past 8 though or you won't find them reliably. 10-15 should do.
Grab those and hang them up on a drying frame. You may need the Hunting skill to build them. After a while they'll turn into random seeds you can plant.
If you're unlucky like me, someone will come and steal your drying seeds until you claim the land around your frames
