Go to see plays in your area. One of the things you will lose, as you gain experience and awareness in stage lighting, is your ability to just "watch" a play, oblivious to its design elements. Begin by studying the quality of light on the scenery, as well as on each actor as they take the stage. Watch how the designer enhances or clashes with the costumes and with what colors they interpret different locations and times: sunset in a dark drawing room or night outside the palace, for examples. Every time you sense a lighting cue, look up to see which instruments are changing. You may find that the gel color coming from the individual lights changes dramatically when mixed upon the stage.
Get to know lighting designers in your area. By now you should have encountered at least one designer whose work may have inspired or impressed you in some manner. Arrange to meet with the designer and ask him questions about the craft and how he learned the skills he's gathered. Many designers receive education for their chosen profession, and while Yale, Harvard and NYU tend to put out the most prolific number of designers each year from their drama departments, the smaller, more off-the-beaten-path colleges and universities may offer even more personalized instruction. Decide what interests you the most about stage lighting and pursue it, whether it's the effect of mood and ambiance that certain colors and angles can create, or just possibly special effects.
Try your hand at designing. A great deal of professors and stage designers will recommend this method, as there are few other ways to learn the intricacy and artistic potential of lighting design. Seek out smaller community theatre productions in your area, and offer your skills as a lighting technician. It is likely that you may have to volunteer as a stagehand or electrician, or to do some other more "grunt work"-oriented task, until you build confidence and gradually build your way up to designer status. Always ask questions and seek ways to better understand the impact of lights on the mood and setting of a visual environment. Remain humble, and you will find yourself learning more than you bargained for in no time.