Discover what she finds funny. Before you start dropping jokes that may bomb, take stock of your girl's sense of humor. As you head out to movies together or engage in conversation, take mental notes as to what makes her laugh. If you learn that, for example, political jokes are what she finds most hilarious, telling jokes of this type is likely your best path to success.
Build your catalog of jokes. Read some joke books or peruse some joke-containing websites, giving yourself a broader selection of jokes from which to choose when selecting the best ones for her. As you build your catalog, focus specifically on the types of jokes she seems to enjoy most.
Perfect your timing. Even the best joke can fail to produce a laugh if told at the improper time. When getting ready to tell a joke, wait until the right moment, selecting a time when there is a lull in conversation. Also, make sure she can clearly hear you to ensure that she doesn't have to strain to take in your joke and that you aren't interrupting her with your joke telling.
Establish some inside jokes. Inside jokes are an effective way not only to make your girl laugh, but also to build intimacy. If something funny happens while you are on a date, or you tell a joke that has her rolling in the aisle, make this part of your inside joke collection, reminding her of the funny incident from time to time, or restating the hilarious punchline to remind her of the successful joke you told.
Pay attention to humor flops. Even with the best planning, you are bound to tell a joke that just doesn't work from time to time. Be aware of these humor failures, taking note of what didn't seem to work. By identifying what failed to work in your humor attempt, you can increase the chances of avoiding these missteps in the future.