I'm not sure if you can use "Please" in a stand-alone context. But you can definitely use it in conjunction with other verbs. Please, in these situations is "請", pronounced qing3.
請幫我 (qing3 bang1 wo3) = Please help me.
請告訴我 (qing3 gao4su4 wo3) = Please tell me.
請便 (qing3 bian4) = Please make yourself comfortable.
Thank you is simply. 謝謝 (xie4 xie4). I've been told not to attach a 你 on the end of the phrase by a couple of people, or at least not to do so very often. So I guess say 謝謝你(xie4xie4 ni3) sparingly. Just stick to 謝謝 =P
Hope this helps!