There are many methods. One is to use a pitch pipe, and by plucking the corresponding string, tune the guitar by ear. This is not the easiest method, but it is definitely a great way to learn.
The other method would be to use an electronic tuner. This method is preferred by guitarists as it is usually faster, and more accurate. Your Ashbury acoustic guitar comes with a pitch pipe to assist in proper tuning. The directions are included in your owners manual. Most Electric models come with electronic tuners, and again the instructions are included.
On our main site we have many tuners, pitch pipes and accessories to help – Click here to check them out

Aroma Clip on Bass & Chromatic Guitar Tuner is just one example of the range of tuners we have on offer