Touch the tip of your tongue to the bottom of your upper front teeth-- in other words between the upper and lower incisors, not quite sticking your tongue out, but touching only the two top incisors.
I've noticed many beginners try to make this sound with their tonguetip too far forward, and touching both the top and bottom teeth. There has to be a gap under the tongue for the air to flow through.
Since only the top teeth are involved in another fricative consonant, /f/, and touch only the lower lip, it is a "similar" consonant. Alternating between /th/ and /f/ can be a useful exercise-- the tongue and lower lip take turns touching the upper teeth, and the air flow can be maintained without interruption.
The voiced th or edh /dh/ is the same, but you voice it, vibrate your vocal cords. Again, the voiced form of /f/ is /v/, so try the same alternating exercise as above.
There are actually two English sounds for "th", a topic discussed, in part, here. Here is a site supplied by geostan a few days ago that lets you hear words spoken in different languages by native speakers.
I hope you get the sound right soon, but here's an advice on how to "fake it" passably until you learn the sound. When I was still beginning to learn English, I would pronounce the voiceless "th" as an [f] (fing, fink, etc.), and the voiced one as a [d] (den, moder, etc.)
A good guide I use to teach my students successfully: Try to keep the voiced and unvoiced /th/ and /dh/ close in sound to the /f/ and /v/ you see in Brazilian, only use the tip of the tongue touching the front teeth gently instead of the lower lip. Presto! It sounds perfect.
I actually think it can be a good idea to stick your tongue a little out of your mouth when learning the voiced th (there, that, the, their) because it reminds you to really pronounce the sound clearly, which is important.
In terms of the unvoiced th (think, thank, thought) which I figure might be a bigger problem to pronounce(?) remember to leave enough room for air to blow out the sides of your toungue briskly and then voice a vowel to stop the sound in a crisp manner.
<< Moderator's note: This question has been added to an earlier thread. >>
I have two questions about the 'th' sound.
First, I was taught to say the 'th' sound, both voiced and unvoiced, by 1)sticking my tongue out (so that I can see my tongue when I look at the mirror) and 2)exhausting air. I notice this movement when people say words such as there, that, then, think, thing, thank etc. However, I do not see such movement when people say words such as something, anything, month, both, with, etc, which the 'th' is placed in the middle or at the end of the word. So I am wondering, do we handle the 'th' differently when the 'th' is in different position?
Second, I want to know if I need to make my mouth a bit rounder when saying the 'th' sound? As I see some native speakers (not many though) will make their mouths a bit rounder, like the shape they do when they are going to blow out candles.
Sorry for this late reply as my computer has been down for a week and I have just managed to fix it.
Thanks you for your advice!
By the way, I find the website provided by duckie so useful as it dissects the pronunciations of all consonants and vowels to a very detailed way.
But I have one follow up question, so can native speaker see their tongue when they are saying words with th sound?
My observation is sometimes I can, but sometimes I cannot.
I am not sure if they just speak so fast that I cannot see their tongue movement, or they in fact have never jutting their tongue out of their mouths.