Answer:
First of all, be warned: adjusting the guitar truss rod incorrectly may ruin your guitar permanently. In fact, it can break the fretboard. Yet if you take precautions and follow directions, you can avoid the worst.
You want to check, however, if your acoustic guitar needs truss rod adjustments. Look down the fretboard from the bottom and top with your eyes at horizontal level with the fretboard. Examine if the neck is bowed (meaning curved). If the neck is up-bowed, down-bowed or completely straight, you should adjust the truss rod.
You can usually adjust the truss rod of the guitar by adjusting the tiny little hole around the sound-hole of the guitar. It is usually by where the fretboard and the sound-hole meet.
Take caution and loosen/tighten the truss rod depending on the situation. For most guitars, you need to turn the rod clockwise to tighten and counterclockwise to loosen. Some guitars, surprisingly though, are different. Please take caution.
If the neck is up-bowed, loosen the rod. If the neck is down-bowed, tighten the rod. Most guitarists recommend, however, that guitars should have a slight down-bow. Guitar necks' being completely straight is not the best idea; remember to make the neck slightly down-bowed.
Good luck!