there are 2 pronunciations:voiced and voiceless . voiced is found in the words : the , then, that, this , there, these, those: in the middle of a word: bother, mother, father, rather, breathe.... but is not usually at the end. Make the sound by putting the tip of your tongue between your teeth and blowing out while making noise. Voiceless can be at the beginning or middle or end of a word:three, thin, thick, through, thought , breath, faith, both, math, bath, wreath,mathematics, pathology, pathetic...Make the sound by putting the tip of your tongue between your teeth and blowing out without making noise.

I know many people that lives in London but don't pronunciate ''th'' english style, so don't worry. Anyway, you need a movement with tongue, it's difficult to explain, but the tongue will go between teeth in sweet mode.