What do I need to get to replace my failing drive on my Toshiba Satellite A105 Laptop (approximately 3 years old)?
The Toshiba support page for the model A105-S4374 claims that the laptop uses a Serial-ATA (SATA) hard disk drive; 9.5mm height.
These are pretty easy to find. I think the 9.5mm height drive is the standard size 2.5" wide notebook hard drive.
With regards to taking apart and replacing the HDD, you might try a google search:
The first result looked promising.