Th makes a voiced or unvoiced sound, depending on the word. The only difference is that one is voiced, and the other is unvoiced.
To make the voiced th sound, open your mouth slightly, and make a vowel sound. As you’re making the vowel sound, slowly bring your tongue towards your teeth. When the sound changes, and your tongue starts to vibrate, you are making the th sound. The tip of your tongue will be between your teeth, almost touching your teeth.
This is a soft action; don’t do anything forceful when saying the th. Also the th is not a short sound; make it somewhat long. (The th is a sound that is possible to hold continuously, as long as you have air in your lungs.)
Some sentences to practice the voiced th sound: (“with” has the unvoiced th sound, but the other examples of th in these sentences are all voiced.)
Their mother was gathering the clothing together.
They’ve had a lot of bother with the weather.
They’d rather gather those berries with their mother.
There’s their brother, together with their father.
Therefore they’d rather go together.
They should ask their father or their mother.
There is another feather over there.
They’d rather bathe with their clothing on.