Hi. I am new here, math and physics undergrad. I have found this website to be very useful. Thanks to all the submitters who have helped students like myself in the past.
In order to help you start, the following are the three main sections in PF. You should explore them well:
1. The main forums, that you see in the first page.
2. Insights Blog, available at the top row (header section). In short, they have been described as "wonder of the web". The blog contains articles that help in learning topics to a great extent.
3. The Videos, also available in the header section. The media gallery now has some great videos and lecture courses in almost every field in science, and they provide an audio-visual learning opportunity.