If you have a block of text you wish to indent, then rather than pressing tab at the beginning of each individual line, select the entire block of text and press Cmd-] to indent it.

The indented string will be the same as your Tab key would indent. This can be configured in the preferences pane.

You can unindent in the same way with Cmd-[

The indent size may be customised using the Editor tab indent size option in the Editing Aids pane of the Preferences window. You can choose a tab or number of spaces in {1,2,4}.