/f/ is a voiceless consonant sound.
The /f/ sound is produced by placing your top teeth against your lower lip like this and releasing the air out like this /f/.
I would say the most common challenge with this sound is a tendency to try to make it without enough tension...
...between the teeth and the lower lip, so the sound lacks energy.
Now, let's practice.
Here's a nice exercise that will help you clarify your /f/ sound while also practicing your front vowel sounds.
Let's try this together.
/f/, /iy/, fee, /iy/, fee, if.
/f/, /eh/, f, Fe, f.
/f/, /eh/, f, /eh/, f.
/f/, /ah/, f, /ah/, f.
And now, let's try some words.
Fan.
Fill.
Felt.
Laughing.
Sniffle.
Fluffy.
Goof.
Stuff.
Half.
Try this out as you practice on EnglishCentral and good luck.