Ctrl+A, Ctrl+M would be the fastest way. Or you can Ctrl+A and drag the
First Line Indent marker on the ruler (this gives you more control). Or you
can Ctrl+A and set a first-line indent in the Format Paragraph dialog. The
best way, of course, would be to apply the indent to the style, but if
you've used the same style for the entire document, it's probably Normal,
and I wouldn't advise changing that.