Just select the text you want indented and hit Tab. To un-indent, select the text and hit Shift+Tab.
If you're in "Normal text file" mode, this will give you tabs (\t
characters). If, however, you're in Python mode (which you enter either by saving as a .py
file or by selecting Python from the Language menu), it should default to using four spaces instead of tabs. To configure the way tabs work yourself, go to Settings → Preferences → Tab Settings (for Python, this should default to "Tab size: 4" with "Replace by space" checked).