A very good site to know about is WEB ELEMENTS. It contains a lot of information about the elements and is being expanded all the time.
Another great site is the Sheffield Chemdex. It contains large resources for students and even has some links to university chemistry departments.
The site ABOUT:Chemistry has lots of information. Make sure you check the toolbars at the left of the page. That is where they organize the topics.
This site here is not quite as all-inclusive as some of the ones above, but it is written specifically for HS students and younger so it might be a place to go to first to see if they have the topic you need help on. It is www.chem4kids.com
A good site to see structures of molecules is www.chemspider.com