Try counting from one to 10 on the mic. Repeat.
Or try being like a computer, and just stick to ones and zeros, in any sequence you like. We call this variation “Binary” it can go something like this: 01001101 01000011. . .
If you’re having trouble saying anything on the mic this is something easy to get started on. You can play with how you say the numbers around the beat. Then mix up the sequence. Eventually as you get bored of the numbers, you can start adding words and phrases and rhymes. Before you know it: freestyle!