If you speak Castillian Spanish (as in Madrid), the "th" sound in English is very much like the "c" in Spanish in words like graCias, fasCinante, etc.

There are MANY videos online about how to pronounce TH. :) Here's one: https://www.youtube.com/watch?v=VUAYmTnWaCY When you practice in the mirror, make sure you can see your tongue as you pronounce the sound. If it's behind your teeth, then you're doing it wrong.