However the first time you register, you'll need to use the unique link for to your school community, which hopefully has been published on your school or PTA website. They should have a PTAsocial button for you there, so you can either register or log in. You may also be able to put your school name and PTAsocial into a search engine and find it that way.
After the first time you log in on any of your usual devices (phone, tablet or home computer), it's a good idea to save the webpage as a shortcut, bookmark or home screen icon.
On your phone or tablet, "save to home screen" so it looks like an app that you can just click and open. If you set up you phone or computer to accept cookies, you should not have to log in again next time (if you don't specifically log out).