Tutorial : How to make a Tree Farm.

Dirt blocks 2 metres apart. 

First place some dirt/grass blocks two spaces away from each other.

Torches on the sides

To speed up the growing, or if you are growing indoors place torches on each sides of the dirt.

Saplings on top. 

Place a sapling on top of each dirt block.

All done!

Wait a couple of Minecraft days.

(Don't worry if your trees all don't grow at once, it's random.)