My letter is one line too long and it goes to a second page. How do I put everything on the same page?

I frequently make use of character spacing to make text fit the space. Decreasing the spacing by 1-3% isn't noticeable. You can apply the spacing to a single paragraph, multiple paragraphs or the whole document. Click the down arrow pointed to in this screenshot (Pages 12.2.1 in Monterey, macOS 12.6.3)

Yes. One option would be to make the margins slightly smaller so that there is more room for text on the page. Click the "Document" icon at the top right of the window to access margins.