The usual method is to insert sections breaks (next page or continuous). You
can specify the number of columns independently for each section. So from
your description, you have page one, formatted for two columns. Then insert
a next page section break: that gets you to the top of page two. Format that
section as one column. Type your paragraph. Now a continuous section break,
format for two columns, and go on from there.

An alternative, if the paragraph at the top of page two is independent of
the text in the two-column layout, is to put that paragraph in a textbox
that stretches the full width of the page. Format the body of the document
as two-column: on page three the columns will start below your text box.