One option for tuning your guitar is using an online guitar tuner.
There are quite a few free online guitar tuners available. These are web-based tools that allow you to generate the tones of each of the guitar strings, and use these tones to tune your guitar by ear.
I prefer to use an electronic tuner that I can connect my electric guitar to directly and ensure a fairly accurate tuning job. However, if you don’t have an electronic tuner available, and you have a decent ability to tune the guitar to a reference pitch, using a free online guitar tuner is certainly a viable option.
Free tuners come in two flavors: web-based only, and downloadable. Below is a good, if basic, online (web-based) tuner. Just click on each string, and then tune the respective string to the reference pitch.
Here's How to Get a Free Downloadable Tuner
In addition there are software utilities that may be downloaded to your PC or laptop, so you will not need an internet connection to use them. Some of these utilities may be downloaded and used for free. The best one I’ve found is one from Gieson Interactive. It has a really cool interface, and some great features like an auto-advance switch, guitar or tone setting, and an adjustable delay between strings. This tuner can be used as a web-based tool, and also downloaded free of charge. If you can tune by ear to a reference tune and have access to a computer, this is certainly a cost-effective option for tuning your guitar