Many modern guitar players are fully dependent on a portable electronic tuner to keep their guitars in tune. These are usually a good choice since they are affordable, easy to use, and highly accurate. However, many people have grown so dependent that they cannot tune their guitar without this accessory.
It turns out that tuning your guitar in other ways is not really too difficult. One good approach is to use a reference instrument that you know is in tune. Traditionally this meant a tuning fork that would match one string on your guitar. Now, it's just as easy to use a piano or synthesizer. This can take a bit of practice since matching the two difference qualities of sounds can be difficult at first: a guitar sounds different than a piano, even at exact same pitch.
You can also find free guitar tuners online. Typically they do not listen and tell you how to change the note. Instead, they play a recorded file of each of the 6 strings for you to tune to and match.
Finally, you can actually get your guitar mostly in tune without any separate tools or resources. You may have noticed that the strings on your guitar overlap: this means that 2 strings can play the exact same note. You can use this fact to fret the lowest string at the fourth fret and then play the second lowest string: they should be exactly the same. If not, adjust the tuning so that they are identical. Then, use the similar relationship for all the other strings so that all strings are in tune with each other.