God=kamisama
Thank you= arigatoh gozaimasu

Whenever someone prayed at the church I attended in Japan, they almost always started off with a phrase like this.
感謝いたします, literally "I'm grateful," is commonly used in prayers. Also, 神様 was often prefaced with something like 天にいらっしゃる父なる〜 "Our father who art in heaven...," or 父なる〜 "Father God..."
Of course if you're talking about a different god, this may not be appropriate...