First, make sure you write it down! Was at a wedding years ago the best man got up in front of everyone and froze. The guy never recovered and just left. Worst part was he bragged for weeks how good his speech was. Don’t be that guy!
Write down several different speeches. See what you like. Best speeches are honest, and from the heart, but don’t forget to get the odd shot in as well. Find someone you trust, read the speech to them so you’re comfortable with it. And don’t make sure it’s not too long.