Step One - Ear Training
Most people think that, in order to figure out songs, you have to really have been born with a great ear for music. This isn't an accurate assumption... almost anyone can train their ear to be able to figure out simple songs on their guitar.
At the most simple level, you, as a future of transcriber of guitar music, will need to be able to listen to a song, and hear when the chords are changing (you'll also need to know what the chord is changing to, but we will use deductive logic to help figure that out). This is a relatively simple task, which you can practice whenever you listen to music. The next time you have a song on, try and identify when the chord changes. Keep at it, and you'll have it in no time.
It will also be helpful, when transcribing, to hear the differences in chord types. A major chord sounds quite different than a minor chord, and being able to hear the difference will be beneficial. Thankfully, there is some excellent software available to help musicians learn to identify chord types, and practice being able to distinguish between them. Download the free trial version of the software EarMaster(Windows) to aid in learning major and minor chords. Move at your own pace, and try your hand at identifying intervals too. Knowledge of basic intervals (defined as the distance between two pitches) will help you better be able to figure out guitar riffs, amongst many other things.
Try to make ear training part of your routine when playing guitar. In very little time, you'll be able to pick out basic chords right off of CDs.
Now, let's make sure we get our guitar in tune with the song we're trying to learn.