Iam going to leave Beijing, by Garuda to Jakarta, do I need to pay Airport Tax? If any how much?

As I know, the Airport Construction Fee (Airport Tax) has been included in your air ticket fare, so you don't need to pay that again.

Well, your tour guide can just recort you to the airport, and then you need to walk yourself to the check0in counter for boarding. Don't worry! The landmarks will be very clear for any procedure and the staff may also speak English.