Start up disk space
On the Desktop, click on your Hard Drives icon, do a Get Info and it will tell you the amount used & amount of free space.
You need to keep at least 20%+ of it free for best operation.
You need to move some stuff off to another HDD or CDs/DVDs.
How much free space is on the HD, where has all the space gone?
Disk Inventory X...
The issue will be more complicated i you're using Filevault.
Please ask if you need more help or have questions.