You will not be able to master guitar scales without learning the basics, which, in this case, is represented by the major scale. As various scales have their starting point in a certain note on the major scale, it is quite obvious why you need to study it first.
The major scale has notes and intervals. By getting to know it to perfection, you will ease your way towards learning other, more complex, guitar scales.
How to start playing guitar scales
A little bit of practice can get you started really nice. As the various scales that can be played on a guitar can deliver various sounds and various emotional effects, it serves to get familiar with these effects, before actually starting to attack more complex pieces. Some of them can sound macabre, while others are cheery. Obtaining the right effect is what you should be after.
The Lonian scale
The first one to study is the Lonian scale. This is, actually, the same with the major scale, so, if you have already taken after the aforementioned advice, it should be quite easy for you to play around with it. This one is best fitted for happy lines, and it is mostly used in jazz, country music, and rock. As you can see, just by mastering the major scale, you are able to play a wide variety of music genres.
The Dorian scale
In case you are more interested in playing dark tunes, you should start focusing on learning the Dorian scale. This one is used for delivering dark, depressing sounds. While it can be identified for the most part with the minor scale, it is worth mentioning that the Dorian scale has a particular twist that makes it very useful for obtaining disturbing sounds. For gothic rock and other dark sounding genres, this is the scale you must learn.
The Lydian scale
This scale is tributary to whole tones and it is mainly used for playing the dreamier parts of country music pieces, as well as rock and jazz. For those that are more romantically inclined, this one is a must learn pattern.
There are other guitar scales to learn, as well, such as: the Mixolydian scale (used mostly for blues pieces), the Aelion scale (for sad pieces that need to evoke such feelings in the audience), and the Locrian scale (used mainly by heavy metal bands because of its particular dark sound). Regardless which one you choose to learn, do not forget to learn the basics first.
Learning guitar scales will open your lead guitar playing horizon. You will be able to improvise and create melodic partitions over any chord progression.