First define the function f. The Basic Math Assistant palette has a button to "Define function" (open the palette from the Palettes menu and click on the Advanced tab of the palette in the Calculator section) that inserts a template for defining functions in Mathematica. As you fill in the template remember that all Mathematica commands start with a capital letter, and square brackets are used to surround inputs to functions. Evaluate the cell by clicking on the Enter button on the palette, pressing Shift+Return, or the Return key on a number pad (there is a tooltip for the Enter button on palette that reminds you of the keyboard shortcuts).The correct form to define f is
f[x_] := 2Sin[x] + Cos[3x]
Once f is defined you can evaluate f, f', or f'' for any input. To graph both f and f', use Plot[{function1,function2},{var,min,max}]. A template for this can be inserted into the notebook using the Basic Math Assistant palette by opening the Basic Commands area of the palette, clicking the 2D tab, selecting the first item in the More drop down list under Visualizing Functions, and filling in the template:
Plot[{f[x],f'[x]},{x,0,2Pi}]
The greek letter for Pi can be entered using the Basic Math palette, or pressing the three keys Escape p Escape (see the tooltip for the pi button on the palette).