are there instructions that I can download on how to replace a HD on an ibook and will a HD from my old ibm thinkpad T30 work or does apple use different size HD
Which iBook?
If it is the white iBook G3 (icebook) or iBook G4, the process is fairly difficult - 2 - 4 hours work and meticulous tracking of many dozens of very small parts.
The Thinkpad hard drive should work IF it is a 9.5mm tall drive -- some older laptops used 12mm drives and this may not fit.
Think first how you are going to get your data transferred to the new drive. An external Firewire or USB case is usually the answer. Back up your data FIRST.