It is difficult to explain how to pronounce a certain sound, the best way to do this is by imitation. However, I can tell you that in most English accents, the 'th' in 'something' is a friction sound that is made with your tongue close to the cutting edge of your upper teeth. You have to blow air in this position so that you make a slight friction sound. It is completely voiceless, which means that if you place your finger against your throat while you pronounce this sound, you should feel no vibration.
I hope it helps.