Let's start with the shape of the mouth.
For both sounds,
we put our tongue between our teeth
and push air out between them.
The difference is that
the first one is voiced,
which means we use our vocal cords
and the second one is unvoiced,
which means we don't use our vocal cords.
OK, try this:
Voiced - vibrations.
Unvoiced - no vibrations.
Can you feel the difference?
Now let's practice with some
common words you already know:
'this', 'thing', 'though',
'through', 'mother', 'birthday',
'clothes', 'fourth', 'bathe', 'bath'.
And of course it helps to
put them in a sentence:
'I never sunbathe, but I love a bath.'
But remember to say them
out loud so you're practicing
the pronunciation.