There are basically two pronounciations for "ch", which one applies depends on what other sounds there around, and especially before, it. You have the "light" vowels (and Umlaute) like I, E, Ü, and the "dark" vowels like A, U, O.
When "ch" follows a "light" vowel, it's a dental sound, little more than an aspiration of the vowel, examples: echt, ich, Früchte.
After dark vowels the "ch" is pronounced at the back of your mouth (think of that weird french "r", only that it's voiceless), examples: Macht, Bucht, kochen.
Edit: I looked around and found a page that could interest you, lots and lots of examples for how to pronounce certain sounds in German.
http://www.utils.ex.ac.uk/german/pronounce/