Unless there is some physical disability that is driving your decision to use dictation in Pages, I recommend that you learn how to use the Pages application first without the additional complications that dictation will introduce. You will need to purchase a quality microphone if your Mac has none, or a webcam and use its microphone. Dictation does not support telepathy… 😉
Start with this guide: Pages User Guide for Mac - Apple Support
It contains this information: Use dictation to enter text in Pages on Mac - Apple Support
Anything that you speak into Dictation will travel to and from Apple's servers in an attempt to convert your spoken word to text. If you have a slow Internet connection, this will make things much worse. Any pause in speaking may throw Dictation into believing you are done and that can be an issue for you.