Setting up the action on your guitar is always the beginning of the exciting world of guitar repair and maintenance. Although these first steps can be scary, it's generally a quick and easy task that addicts people to guitar repair for years. The action of the guitar is simply how high the strings are above the 12th fret. When adjusting the action on your Yamaha guitar, your main focus will be on the guitar's truss rod.
Remove the Phillips screws that hold the truss rod cover down, using the Phillips screwdriver. It's located on the headstock, directly behind the guitar's nut. Depending on the model Yamaha you have, there will be one screw or three screws holding down the truss rod.
Turn the truss rod clockwise to add additional "neck relief" or counterclockwise to lessen it. If the strings buzz when played, then you will want to add relief. If it feels like you have to press too hard when playing, then you need to remove relief.
Retune the guitar to pitch and test its playability. If it needs more adjustment, then repeat Step 2 until guitar action is to preference.
Reattach the truss rod cover using original screw(s) and a Phillips screwdriver.
If your truss rod requires more then two full rotations of the adjusting nut, then the guitar may have additional issues. Inspect for a broken truss rod, cracked neck pocket or incorrect bridge height. When adjusting a neck using the truss rod, it is common for the neck to have a delayed reaction, which means you could go back the next day and it will need another slight adjustment. This is common and should be expected.