1. Learn a few basic words and how to pronounce them, then it will get easier e.g. окнo, лето, спасибо, однo (akno, lieta, spasiba, adno). 2. Get a phrase book that shows you how to roughly pronounce them; or a bilingual dictionary that shows word stress marks (e.g. Oxford Beginners Russian-English dictionary) there o is pronounced 'o' if it is stressed and shown with a stress mark (acute accent), and 'a' if it is unstressed. Then practice !! 3. Don't believe Russians that it is always pronounced 'o'. It seems so to them but we are only learning !!

Commonly, if "о" is a stressed vowel it is pronounced as /о/, if unstressed - it is pronounced nearly like /а/ (like in words "хочу́", "замеча́тельно"). Here is an article on english wiki: http://en.wikipedia.org/wiki/Vowel_reduction_in_Russian#Back_vowels

? Как пишется, так и читается. "o" sound like "o", but if you will say with "a", people will understand you.

Btw, I still don't know to mark off a letter "o" but it sound "a"? How we know it?