Here is what I tell my newbie friends...
First off, don't stare at or smile at a girl that you don't want to come sit with you. Second don't tell anyone it is your first trip to Thailand and you just arrived.
If a girl you are not interested in asks to sit with you, just smile and say "no thanks". If she sits down without being invited, then politely ignore her. Even if she is hanging on you, don't give her any encouragement. If she asks the inevitable questions (what you name, where you come from, how many time you come Thailand) then just answer politely without asking any questions in return and go back to watching the other girls on stage. If she still doesn't get the hint and asks for a lady drink smile like she made a joke and shake your head no. If you spot a girl you are interested in, point her out to the girl trying to sit with you and ask her to bring the girl over. That gets rid of the one you don't want and you don't waste time with a girl you have no intention to barfine.
Save the lady drinks for the girls that you enjoy sitting with.
Unless the waitress are doing something beyond bring drinks I don't buy them lady drinks. Give them a small tip when you settle your bill if they provided good service.
If you ask or otherwise encourage a girl to come over to sit with you then you should offer a her a drink. But if she asks for one the minute she comes to the table it is a bad sign. I probably wouldn't barfine her in that case. She should be patient enough to wait till you offer it. Just don't make her wait too long.
Realize being overly generous for no reason just encourages the wrong type of girls to swarm you. Save your generosity for the girl(s) that earn it with a great ST/LT experience.