There are seven kinds of seeds available in Farming Simulator 2013 that will grow into crops you can harvest using different kinds of machinery. When beginning a new game, you will already have all the seeds stored in bags on your farm. Each refilling of your sowing machine will cost you money (excluding potato seeders because those can be refilled for free using potatoes available in your storage) at the rate of 600 $ per 1000 litres of seeds.
To plant the seeds in a field, you need:
In the starting equipment, you already have a tractor, a basic sowing machine and seed bags to refill it from. Enter the tractor, couple the sowing machine, approach the seeds bags, and refill the machine. You can choose the type of seed you want to plant by pressing "O." Then, go in a field that is ready to sow. They are easily recognizable, those are the grey fields with no visible stubble on it. The you can start sowing your field, by lowering the machine and starting it. You can hire a worker by pressing the "H" key.
Tip: To save you some time while seeding and to double your yields, you should buy a fertilizer sprayer like the AMAZONE ZA-X 1402, and place it on the front of the tractor while the seeder is on the back, then turn the sprayer on and start seeding or hire a worker. If you hire a worker, you do not need to buy seeds or fertilizer. Money will drain from your account to pay for the seeds and fertilizer the worker consumes.
Note: There are multiple locations on your map where seed bags can be found. This can help you to save some time in case if your seeder runs out of seeds and your farm is too far away.