Do you have a laptop or a desktop computer? In desktop computers, the replacement of an optical drive is rather simple. Either there is some kind of installation instruction sheet shipped with the drive, or you can download it from the manufacturer's website.
so can I get an external one for a desktop and will it affect my dvd driver that is in there now?
If your computer is equipped with USB 2.0 HiSpeed controllers, then you can attach an external drive. Coexistence with existing internal drives is no problem.
One thing you need to know if you don't want to replace the internal drive:
You may not be able to boot from that unit (for example if you need to reinstall Windows). This depends on your computer's Bios and there are also some restrictions by some Windows install discs.
I can not find just a cd writer by itself. is the hp one a good choice?
Plain CDRW drives are out-of-the market. I'd prefer other brands than HP