Make a hoe (n, j) by putting two sticks up the middle on a workbench crafting square and two [plank/rock/iron/diamond], one in the top middle and one to the side of that one.
Use this hoe to hoe grassy tiles, which will occasionally result in seeds popping out. Now, make yourself a little farm plot by hoeing some more grass in a place where you want to grow some stuff. Now plant your seeds in those prepared tiles and eventually you will get a wheat crop. Wheat can be harvested and then made into bread.
Some notes: You'll need to keep animals off your fields as they will trample your crops (as will you, and walking over hoed ground too much makes it revert back to normal mud). The crops will grow best if they're near a water tile, I forget what the range is so play around. But they'll grow without water, just slower.
You can also make mushroom soup but you need to collect mushrooms for that.
edit: to use a hoe you right click on the tile rather than left clicking (which will just dig).
EDITED: 10 Nov 2010 10:51 by X3N0PH0N