To play the D note, pick the open 4th string.
Play the E note with the second finger of your left hand behind the second fret of the 4th string.
Play the F note with the third finger of your left hand behind the third fret of the 4th string.
Here is an example which makes use of the notes D, E and F on the fourth string. As with any new notes, name them out loud as you play until you can do it easily. Notice how the first two bars of this example move between different octaves of the D note.