It's pretty hard to change the HD on a powerbook.
If you don't have the skill or knowledge to find out what type of HD it needs without asking on here, then the best thing is to go to an Apple repair shop and ask them to replace it for you.
Expect to pay about $100 or more for the work involved in replacing the HD. That's on top of the cost of the actual new HD. Ask them to transfer the data over from the old HD as well. And make your own backup too, before you take in your laptop.
I'm not dissing you, it really is a hard job. I know lots about apple computers, but I wouldn't do it on my own on a good quality working 1.5ghz Powerbook.
There's lots of small computer shops that can repair apples - look for something that says Apple Certified Technician or something like that.