The primary criteria for joining PF is true love for science. Once you have that, you'll find PF a very helpful community. Just post your question in the proper section, and members will surely help you as far as possible.
In order to help you start out here, 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 a "wonder of the web". It 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 in almost every aspect in science, and they provide an audio-visual learning experience.