If you want to create a shape the exact size of your preview just double click the Elipse, rectangle or rounded rectangle tool. read more

If you want to change your default black background color, a common way is to make a solid layer with preferred color then move it to the back.

Another, less common, way of doing this is to just change the background color. Hit Ctrl+Shift+B and you will get this window:
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You may find yourself working on a dark or light project with very little contrast. A simple display adjustment can help you navigate through you without having to hunt for those low contrast elements. You will find the "Adjust Exposure" icon to the bottom right of your display window. Changing this only affects your display and will not change rendered output. read more

If you are planning to export an after effects clip with an alpha channel and need to see what is transparent you should set your view to display the transparent area. Just click the toggle transparency icon located under the display window. (Located here) read more