On the back of your membership card, there is the information you need to log on in terms of what part of the site to go to. You'll also need your local unit number, which is printed on your card. If you don't have your ID card, then contact your PTA's membership person to get the information.
Please note, the national PTA Website's members-only area is a paid service. If you're a member, please contact the PTA for any codes and/or passwords.