You speak from your heart. Let them know, how much they mean to you and what you want for their future. There is no wrong or right thing to say, when you are giving a toast. The toast can be either long or short. There is nothing written anywhere, that you have to follow certain rules.
No certain rules but good guidelines: Share a cute, funny or meaningful story about them Share what is unique and special about them & their relationship. Give a "blessing" on their marriage. Doesn't have to be religious.