For PR, boarding sequence is usually the following:

a. handicap/passengers on wheelchair
b. passengers with babies and/or small child/children
c. rear section
d. middle section
e. front section

business class and elite passengers can board anytime when b-e are already boarding


and yes, you can reserve seats in advanced by calling in their phone numbers, but unless you are elite, they usually are busy lines or you'll have long waiting times