Obviously practice it so you know it by heart. I would relax. You are surrounded by friends and this is a really joyous occasion. Half of the guests are going to be drinking. All the guests will want this to be the best event possible and you will get a huge applause regardless of how it goes. If you are worried about introductions then ask someone to help you get started. The father in law could tap his glass and announce the best man speech is starting. Seriously though. Please. Feel happy that your best mate wanted you do this. It’s a special bond. It is his big day. They want you there and they want to hear what you have to say. You really have nothing to worry about.