Familiarize yourself with the equipment, from the light board to the lights themselves (Lekos, Fresnells, spotlights, and more).
Take part in viewing the rehearsals as early as possible, so that you get a feel for the script, material, and performance choices.
Analyze the script and create a color palette for your production on a scene by scene basis, notating within your script colors or preliminary lighting choices.
Even the simplest color palettes can be highly effective--pink for romance, yellow for daylight, blue for moonlight, fear or sadness, red for terror, rage, or violence, and more. Each combination offers more choices, so don't be afraid to experiment with your options.
During rehearsal, notate important light cues in your play script, for use in running the lights during the actual staging itself. Use pencil until the dress rehearsal so that all notes can be changed as needed.
Create a blueprint of your desired light-rig setup for your upcoming production on a piece of simple graph paper. Make a note of where the lights are, where you might want to move them, and of what colors would be most effective (and where).
Familiarize yourself with the light board. These are easy to operate as long as you plan ahead and practice before the show.
Work with such options as faders to add subtlety to your production's light choices from scene to scene. Subtle changes to lighting as the action or conversation onstage evolve can be magical!
Make final notes in your script of specifics like fader levels, channel groups, special effects (like flashes) and more.
Go over these with other tech crew in preparation for performance. Use a highlighter to notate all major lighting changes and cues. Before major changes, "cue" yourself in your script a minute or two in advance of each important moment to come, as well, so that you are always prepared.
Run a "playback" rehearsal late in the play's rehearsal process in order to simply double-check light levels and settings on a scene by scene basis with the director.
The playback rehearsal is a tech "speed rehearsal" that only looks at changes in lighting for your benefit, giving you the chance to set the channel faders and settings in preparation for performance. This does not involve a full run of the play or dialogue.
Maintain contact with director and stage manager as your production goes underway, in case further slight changes are needed.