ตัดมาให้อ่านกันจ้า
You should put the tip of your tonguge (just the tip, not too much) between your teeth (use a mirror, you should be able to see your tongue) and then you force air over your tongue and under your top teeth. Hold this sound for a full second, then say it for shorter and shorter times, then try to put it into words both at the beginning, middle and end of a word.
Also, you must realise that there are two sounds for 'th',
both are fricatives(meaning you make the sound by forcing air through parts of your mouth)
one is voiced (meaning your throat vibrates, or you 'hum' as you say it)
one is unvoiced (there is only the silent hiss of air as you say it)
For more info, please visit this website: .soundsofenglish .org/pronunciation/th.html (add www, remove spaces)
You can find pronunciations for these words in an online dictionary with sound and listen to the difference, and practice it for yourself.
e.g.
(unvoiced) Thick Throat / Without Wrathful / With Hearth
(voiced) Though There / Weather Southern / Breathe Wraith