How to Install new hard Drive?

Can anyone tell me how to Install new Hard Drive? My idea is to either keep my existing 30 gig Partitioned (15 each Drive) Hard drive and add another.
Is it as easy as Installing a CD-RW Drive? Or does it get iffy going thru the Bios and changing stuff? I know it’s easy for people that have already done this,but I have never.
Is there a site for step by step directions?
Advisable to re-format the old hard Drive first,then install the new one….if I keep it the way it is now, is there a chance that the new operating System might get corrupted?

Thank You!