It is a normal feeling to be intimidated and feel pressure to give a wedding toast. I always find that there is so much history that you have with the couple that it is hard to condense it all into a 5 minute toast. Here is a helpful guide to follow when preparing for your toast:

PREPARE: A few weeks before, collect your thoughts and create an outline with bullet points on what you would like to say.
KEEP IT SHORT: A toast should be short and sweet and should not be any longer than 5 minutes.
WRITE IT DOWN: Write down some key notes, but do not read straight from your notecards. Jotting down some key points will trigger your memory if you get off point.
BE CONFIDENT: Stand up tall, talk slowly and scan the room. Make eye contact with some familiar friendly faces throughout the room as you are toasting. This will engage the audience.
PREPARE: You have heard it before, and you will hear it again, practice makes perfect.
FINAL WORDS: Wish the couple a happy future together. Offer advice or pull a quote from a book or song lyric.

Cheers!