Creating a New Document
Go to File > New and select the Web preset. This will set your color mode, units of measure and resolution to the correct settings for web graphics. There are also a number of Page size presets for the most common sizes of web graphics and banners.
Defining The Margins
This is part of the Page Setup, but it’s an important step. After defining the page size, you should choose the inside and outside page margins. Inside margins, because it’s not good to place text or objects near the edge of page ̶ it’s not just about the aesthetics or design visuals ̶ even if there is little difference when cutting and it’s not noticeable that there is a margin around the inside.
To add inner margins,go to the Layout > page setup...
Then Click Document > Guidelines > Presets and Click the User Defined Presets and Check "Margins" setup the value and check the mirror opition then click on the “Apply Presets” button.
If you are using a background, or if an image is near the edge of the page, you should add extra space around the outside. This outer margin is called “bleeding” and is very important because the cut is not always as accurate as expected. This additional margin helps the process of post-printing a lot. You can specify the size of the Bleed when you choose the size of the page. Double-click on the edge of the page, or go to Layout > page setup . In the section “Bleed” you can set the desired distance (usually 3mm or 0.125″). In addition, you can make this area visible by selecting “Show bleed area“.