The correct pronunciation of the two ‘th’ sounds in English can be difficult but, don’t worry! Here’s a simple explanation in one minute!
In which group of words is the ‘th’ pronounced /ð/ (remember, this is the ‘voiced’ sound, where we use our vocal cords)?
1.think, birthday, breath
2.this, other, breathe
Summary here: https://www.bbc.co.uk/learningenglish/english/course/eiam/unit-2/session-59