Voiced and voiceless

There are two ways to pronounce the "th" sound.  Both "th" sounds are linguadental sounds, meaning you make the "th" sound with your tongue and teeth (tongue slightly out between your teeth).  The voiced "th" (mother, these) is produced with your voice on and is typically mastered by age 5.  The voiceless "th" (thing, moth) is produced with your voice off and is typically mastered by age 6.

High Frequency "TH" Books