Give Me a Break!
Hopefully, you are off to a good start and know what type of humor will work for your audience. How often do you need to throw in something fun? You certainly do not want to give the impression that you just tacked on someone else's joke at the beginning because that is what you were expected to do.
Odds are, though, listeners are not expecting a laugh-a-thon; you just need to occasionally lighten up a straightforward talk. Even Shakespearian tragedies have comic relief to make the heaviness tolerable. Too much seriousness can also put people to sleep, so you need to find points to insert something upbeat.
After you decide how to open, just work on your speech without reference to giving it humor breaks. If something occurs to you that seems to fit in, make a note, but just get a rough first draft done so you can see how long it will take to deliver the material you absolutely need to cover within the time limit. Then look at it for potential openings for a little levity, perhaps every five or ten minutes. These do not have to be full-blown jokes — a wry comment or a sarcastic observation will engage the audience.
“Brevity is the soul of wit,” said Shakespeare. Your jokes should be short enough to memorize because reading drains the life out of them. Ultimately, what matters is whether the audience feels that the time taken to tell your joke was worthwhile. When in doubt, cut the unnecessary detail to get to the punch line quicker.
Underscoring an important point you want people to remember with humor is the ideal way to deploy it. It can take an abstract idea and make it concrete. If you say something amusing and it has a graphic element, describing it vividly will help it stick in the minds of your listeners.
Do not use humor that is not in some way relatable to the overall message you are delivering. Everyone has heard too many jokes told to open a speech, which then goes in an entirely different direction. Tacking on that kind of humor artificially will not help you succeed and it may hurt if it flops.