You need to start your speech with a joke.

The logic being that you will relax both yourself and your audience.  This may well be true, but there are some significant downsides.

Is the joke related to your topic?  If not, it will likely confuse the audience and take away from your message.

Are you good at telling jokes?  More to the point, are you good at telling jokes when you are nervous?  If not, what if the joke flops?  Rather than put everyone at ease, you just raised the stakes!

What if the joke flops?  What if it offends some people in the audience?  The benefit of putting everyone at ease is far outweighed by the potential for failure.

Don’t start your speech with a joke – leave the joke telling to the comedians