Another, more portable alternative is to tune to pitch pipes. Most music shops will have in stock a set of six small pipes which are tuned to the open strings of the guitar. If not, they should be able to supply you with a single one, in which case a G is the best note. With this G you can first tune your 3rd string, then tune the others from it by the process of relative tuning.
RELATIVE TUNING
This is by far the most efficient method of ensuring that your guitar is in tune with itself, which for playing on your own, is far more important than any consideration of perfect concert pitch. Relative tuning works by matching the sounds of adjacent strings, as follows: Having tuned the 3rd (G) string to your satisfaction, press your finger down on this string. Then, keeping your left-hand finger on the 3rd string, so that it will sustain the note, pick the open 2nd (B) string.
The sound produced by the 2nd string should be identical with that from the 3rd string stopped at the fourth fret, because you have shortened the length of the vibrating part of the 3rd string by four fret and raised its pitch from semitones. i.e up to the note B. If the 2nd string isn't in tune, then it must be adjusted until it is identical. I must point out at this stage that when counting frets you should not include the nut that is, the bar of hard wood, or ivory which is situated at the top of the fingerboard. All the musical vibration of a string takes place between the natural boundary points formed by this nut and the bridge. You will find both of these parts of the instrument indicated in the diagram on previous lessons.