I always recommend to all WolframAlfa for any level of knowledge. This is a very good resource both for studying the basics of mathematics, and for those who want to improve their level. Also, pay attention to the site assignment.essayshark.com/math-help, because here you can perform a specific assignments with the help of a mentor or find useful information in the blog. Khan Academy can also be useful to beginners, since there are courses designed for any level of preparation. From the books I advise "The Princeton Companion to Mathematics" by Timothy Gowers. It will help to trace the development of modern mathematics, explain the main terms and concepts, to study the basic ideas in the basic fields of mathematics.