The quickest way to get started is to create a new freeform canvas by clicking File|New and selecting one of the default sizes shown in the menu. For more detailed structure you can use the collage wizard
open an existing template using the Poster Wizard dialog. Open this by clicking File|New Poster Wizard. Here you can select one of the existing templates or you can customize your own.
To select one of the existing templates click on the default size you want to use and then select the desired layout that appears. Double click on the desired layout or highlight it and click OK.
The other option is to click the Custom tab and you can create the exact poster you are looking for.