When you first get started on guitar, you'll find out that there's a wealth of content out there. Some good, some not so good. This link post has a great article all about tuning your guitar, and is something every student should check out. It teaches you how to tune your guitar with a piano, with the "5th fret method", with harmonics, and lots more.
It also has a great layout, and has easy to understand pictures. Go check it out if you're just getting started on guitar.