http://www.tanos.co.uk/jlpt/skills/grammar/

As I'm a poor teenager, I can't afford many books. This website (if complete) seems perfect for grammar study. Could someone more in the know tell me if this site covers the grammar points you need to know?

There's no guarantee that it's perfectly complete, but many people rely on it. There's no real guarantee that any of the printed books are actually complete either... it's all assumptions based off of previous tests.

However, you don't need 100% to pass any given level of JLPT, so if you study the grammar points from tanos you should be fine. (Be sure to study or review the N5 and N4 grammar points even if you're going for N3... items from lower levels aren't duplicated on higher level lists.)

Now, whether or not the explanations actually suffice or not is another question, but since many people study from tanos+jgram a carefully phrased google should bring up questions on one or another Japanese learning forum. If not, well, you can always ask yourself.