How can I set a accidental to a note?

When entering notes with the mouse, the height position is always calculated for the currently key. In a G major piece, it will be always an F#. To get a C# in a G major piece, enter first a C. As long as the note still is selected (blue), use the keyboard keys arrow up and arrow down to shift the note in half steps to the desired position.

The tutorial shows this procedure in detail.

PS: Depending on key or transposition a note can be displayed differently than intended. To change this, select the note and click on the button "Change accidentals".