My phone is an Android Evo, and it keeps telling me I have low storage. I have removed everything - facebook, email, messages, text messages, and pictures, but it's still low.
This should list available space and the size of used space for apps, pictures, videos, music, downloads, etc.
You can use this as a guide to go through your storage and find files you can clean up.
Once you have figured out what is taking up a lot of space, then you can ask how to clear up a particular item.