I was nervous too when I first went to the bars in BKK. But this forum really help and you will realise that when you are in the bar itself, the atmosphere and the cute boys on stage will melt away any nervousness you have.
From my personal experience, i find that most of the mamasans in BKK are also friendlier and perhaps abit more honest than those in pattaya. Just be alert to any fishy activities and you will be fine.