Light up your performers. If the audience can't see the performers, it doesn't matter what is happening on the stage.
Pay attention to the script. It may not be explicit, but most lighting needs are written in the script. If a scene takes place outside on a sunny day, you will need to create bright sunlight. If it's a rainy day, you will need to bring the lights down to a dull gray.
Use lighting to direct the audience's eye. If there is a conversation taking place downstage, but upstage an actress is sneaking onto the scene to steal an important prop, a well-placed spotlight can draw the audience's attention to the event taking place that they otherwise might not have noticed.
Choose the right colors for each production. Dark lighting may be more appropriate for a violent drama or a play that is supposed to produce scares.
Know your equipment. Don't design any production without a visit to the theater. If you are renting the space, they may not have every lighting fixture that you are used to using. Find out what equipment is available to you and design your lighting with that equipment in mind.