You will have to make a Cultivator at the Forge, it needs 5 core wood and 5 bronze. With it equipped, right click to bring up its menu, and the 1st icon will be Cultivate, select it and while aiming at the ground you can till the land and prepare it for seeds. the menu also has all the seed options you currently have unlocked, like carrots and the differnt kinds of trees. more seeds and stuff will be available to you in the cultivator menu as you progress too =)