how do i improve the speed on my mac book 2008

More memory is always beneficial, but before spending money for more, determine if your system's performance is being limited by the lack of it.

Read: Using Activity Monitor to read System Memory and determine how much RAM is being used (OS X Mountain Lion and earlier)

Addtional RAM can improve stability but there are other ways speed up your Mac ...

And make sure there's enough free storage space.

Click your Apple menu  top left in your screen. From the drop down menu click About This Mac > More Info > Storage Make sure there's at least 15% free disk space. Less can slow your Mac down.

Make sure your Mac's software is up to date. Click Software Update from your Apple  drop down menu.

Repair permissions.

Launch Disk Utility located in your Applications > Utilities folder.

Select the startup disk on the left then select the First Aid tab.

Click: Repair Disk Permissions

This can take awhile so don't be alarmed.

Restart your Mac after Disk Utility is finished

Reindex Spotlight: Spotlight: How to re-index folders or volumes

Check to see if the startup disk needs repairing.

Launch Disk Utility located in HD > Applications > Utillities

Select the startup disk on the left then select the First Aid tab.

Click: Verify Disk (not Verify Disk Permissions)

If DU reports errors, restart your Mac while holding down the Command + R keys. From there you should be able to access the built in utilities in OS X Recoveryto repair the startup disk.

Make sure to back up all important data first before using OS X Recovery.