I DON'T GET IT
If you are having trouble understanding your homework you should always contact me, But if you want more help, these websites might be able to help you.
The Khan Academy
For high school students this is a fantastic source of video tutorials on almost every topic in Biology, Chemistry, and Physics. Also a good source for math help.
Brightstorm
I don't find Brightstorm's videos to be quite as helpful as those from the Khan Academy, but they are shorter and may be more engaging than the Khan Academy videos in some cases.
Science Gems
In this website a teacher named Frank Potter has assembled a series of useful websites for each grade level in a variety of subjects. Just click on the class (Physical Science, Earth Science, Life Science, or Math) that contains the topic you are interested in.