To get wheat you need to harvest planted seeds;
or you can get wheat a potentially much quicker way by crafting bonemeal from bones dropped by skeleton enemies, these instantly grow a single crop of wheat (thanks to RadiantViper)
Seeds are left after hitting tall grass, not always though, maybe around 25% of the time.
For the seeds to grow, grass or dirt blocks must first be tilted. To till dirt and grass blocks use the Hoe tool and press LT, you should then see the texture of the block change. Ensure the block is hydrated, it goes a darker colour when surrounded by water, and now plant the seeds.
To speed up the growth of the crops, ensure there is a decent amount of light(ie. No shadows)
Make sure you do not to trample your crops by walking over them, and I suggest fencing the crops off to avoid animals trampling on them.
The crops grow around a metre high (the size of one block) and turn yellow when ready to harvest. If the growth has been good, enough sunlight and enough water, youshould get a couple more seeds to plant and wheat. If you plant half a dozen in one go you should get enough wheat to be able to turn it into bread at the workbench.
.Solution originally provided by JimboJambo86.