cant use italics

Please help I am writing an essay and the italics button is greyed out. When I try to select italics in styles etc it show as ticked but the text doesnt change. Please tell me where I am going wrong before I tear my hair out! Also how do i do a screen shot?

If the font you are using doesn't offer an Italic set, then italics aren't available.

Screen shots can be done using command-shift-3 (for the whole screen) or command-shift-4 (a selected area of the screen) or command-shift-4-space (a particular window only).