Find a patch of good soil. (It doesn't have to be yours but if the person who owns it finds out you're planting stuff on their soil they might get mad.)

The cheapest way to find seeds is to go to a store and take some seeds from some of the display plants, but they might get mad at you for doing that so you can always buy seeds from them instead.

Make a small hole for each seed and place the seeds in the hole. (If you're doing it on someone else's property, make sure they don't see you doing it.)

Seeds need 3 things to grow:

Sun (which is in the sky incase you didn't know.)
Nutrients (which are in the ground.)
and
Water

You need to water seeds so they don't dry out and become ungrowable. Pour water until the soil looks wet. You have to do this every time the soil stops being wet. Though you don't have to do it if it rains.

Seeds take forever to grow so you'll need to wait.

Once the stuff has grown, harvest it and then you can eat it or sell it!

You have successfully become a farmer!