You'll begin using Pagemaker by setting up the initial formatting of your document. If you select File>New you'll get a window called "Document Setup:"

This dialogue box gives you a number of options for formatting your new document. You'll probably want to set the following properties: Page size, Dimensions, Orientation, and Margins.

Under Page Size there are a number of standardized sizes from which you can select; alternatively, you can enter your own dimensions in the Dimension box just below.

Also, note the Options boxes: If you're creating a poster, you'll want to un-check the Double-sided box; you might also want to set all margins to a uniform amount. If you're creating a multi-page booklet or other bound presentation, these settings might be different (you can experiment to find the combination that best suits you).

An appropriate Target output resolution for Pagemaker is 300 dpi.

When you're finished, click OK. A new file will appear on your screen with the dimensions you specified.