My mac keeps saying the start up disk is nearly full how do I fix this

See Here... what-to-do-when-your-hard-drive-is-full.html

and also have a look here... Where did my Disk Space go?

And Here > The Storage Display

Freeing Up Space on The Hard Drive

1. See Lion's Storage Display.

2. You can remove data from your Home folder except for the /Home/Library/ folder.

3. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.

4. Also see Freeing space on your Mac OS X startup disk.

5. See Where did my Disk Space go?.

6. See The Storage Display.

You must Empty the Trash in order to recover the space they occupied on the hard drive.

You should consider replacing the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.

Try using OmniDiskSweeper 1.8 or GrandPerspective to search your drive for large files and where they are located.