Help! I am not understanding Greek phonetics?

I notice that some Greek letters make different sounds at different times. Consonants and vowels alike, at times, when pared together in combinations make the sound of a single letter. The Greek "y" or "gamma" sometimes makes an English "y" sound, and sometimes it makes a sound somewhat similar to the English "g". How am I supposed to know when it does what? Would some one please explain this to me? Or can someone direct me to a web site that explains the Greek rules of phonics? This all very confusing to me. Thanks to any and all who can help.