To create a new Photoshop document, follow these steps:

1. On the main menu bar along the top, choose File>New.
2. Enter the name of your file and choose the size of your document. Notice the second line, labeled Preset, where you’ll see a number of common sizes in the pull-down menu. By selecting one of these, the rest of the sizing will be done automatically. Also notice that A4 print size is among them (near the bottom).

As a general rule, images created for print should be at least 300 dpi (dots per inch); and images for the Web should be set at 72 dpi. The width and height will be set as pixels by default, but you can click and hold the arrow buttons to change the increment to inches, centimeters, etc., in the New dialog box.