To pronounce thinner, place the tip of your tongue at the bottom of your front teeth, and expel air through your lips. For f, touch your lower lip to the bottom of your front teeth and expel air. Both sounds will have a hissing quality, but no voice. (With voice, the comparable sounds are th as in the and v as in very.)
The d in dinner is pronounced with the tongue just behind the top teeth, with voice; unvoiced, the sound with the tongue in that position is t, as in too.
Practice in front of a mirror if it helps.