I was a best man a few years ago. I'm not particularly shy and used to be a teacher so speaking for an audience doesn't worry me too much, but as a female best "man" I had that extra pressure to worry about instead!
Tips:
Everyone is on your side, and they will stay on your side unless you a) speak so quietly they can't hear you, b) go on and on for fucking ever, c) become massively inappropriate.
So - nice and loud. Have a quick practice on the day in the room. Get someone to sit at the back and tell you if you are too loud / too quiet. (I had to do this even though I am VERY loud 😂 because I had a microphone and I'm just used to bellowing so I needed to get the level right).
Timing. Write the speech, practice the speech, time the speech. No shorter than 4 mins (people look forward to it, it's a key part of a wedding, so it really shouldn't be any shorter than this) and ABSOLUTELY not longer than 10. Try to find out the order of the day, if the speeches are before dinner keep it shorter than if they are after - people who are hungry will be a less happy & patient audience than people with full bellies! Also worth finding out how many other speeches there will be; if there's loads that is another reason to keep it shorter.
Resist temptation to tell everyone the absolute worst things you know about the groom. People want a laugh, but no-one actually wants to hear about the time Mikey got so pissed in Malaga he took a shit in a prostitute's hair. Also, the aim is to have a bit of a laugh at the groom's expense - nothing you say should embarrass or upset the bride - which also means there should be zero references to the groom's ex-girlfriends.
Extra tip: props can help. It shows you've put a bit of thought and effort in, and can help land jokes. But it's an extra thing to have to think about and physically handle (especially relevant if you have a mic) so it depends on how nervous and fumbly you think you might be.
Finally - one or two drinks before to steady the nerves but not enough to stray into tipsy. And don't get caught writing your cue cards at the last minute during the reception by the official photographer, like I did! 😳😂