Q: booting off of an external device

Im wanting to partition my hard drive in order to run boot camp, but Im worried about something going wrong and my not having any means of booting off a rescue CD.  What do I do if something goes wrong and my system or drive gets hosed.  In the old days, Id simply boot off a rescue CD and use my system install disks - no drive anymore.  Shoot this computer was gifted to me and I don't even have the system software.  What I do have is two external USB hard drives.  If anyone could please take a few minutes and guide me through today's rescue procedure I would be very appreciative.  Getting old and not staying current *****.  Thanks in advance for any help.  FYI, I'm on a shoestring budget and can't afford to buy anything.