Alright Cindy. You would need to get a install disk for leopard, and run the disk utility from that to really go any further.
But given your symptoms, I'd suspect you're dealing with a failed hard drive. If you want to try to replace it yourself I can try to guide you through it, but you'll still need the install media to even reload the operating system.
I'm not entirey sure what the Apple Store would charge for such a procedure, but they should be able to get you the OS.
I'm not entirely sure what version you're running, but for example, snow leopard can be ordered here:
If you know what version and what like to me look it up I can try to find it. Typically, a SATA hard drive (what that has in it) is usually around 60-70$'s.
In order of things though, I would first get the install disc and attempt a repair, and/or a fresh install. Just in case the damage is software corruption and the drive is actually ok. From there make the determination of whether or not to replace the drive. You can always respond to me at anytime (even after rating), even if its a few days/weeks later you can still reach me again via My Questions
at the top right. Drive failure is one of, if not, the most common issue with laptops, so it's a safe bet that's what's failing out. It would exhibit startup issues, massive speed issues, and very sporatic functionality.
Let me know what you decide and if I can assist further in any way.