I've done best man duty and the top confidence tip is to remember everyone is on your side, they've had a few drinks and they want to laugh.  You only have to be a little bit funny to get a few laughs.

If you can start with a good, shortish gag – once you get your first laugh you'll relax and start to enjoy it.  My mate has longish, floppy blone hair so I opened with "I was a little surprised when Ed asked me to be his best man.  I wasn't wearing my specs and I thought he was Boris Johnson."  Not the best gag ever but got a big laugh, and was obviously specific to him rather than being an endlessly recycled standard best man line.

Unless it's a hugely formal do where you're asked to do something specific in terms of what you say, who you thank etc, don't be worried by the occasion.  Think of it just like telling (reasonably clean) jokes and stories at the pub.  That's fun, right?  Then your speech should be fun.

I was hugely nervous but really enjoyed it.  Absolutely nobody noticed my nerves apart from the photographer, who said he could see my hands shaking when he zoomed in.