to get a farm you need seeds. once you have seeds, you can plant them with $use
seeds can be obtained like any other ‘rare’ item, from crates and scratch cards. different crates / scratch cards will have different odds for giving seeds.
you can see the odds for crates on the crates page
the current seeds can be found on the items page
with seeds you can grow plants, which you can then harvest for items you can make money from or craft things with.