how do i know which harddrive to buy

MacBook Pro (15-inch, Mid 2012) - Technical Specifications

Your particular model of Macbook came with either a standard mechanical 5400rpm drive, or a SSD. Both using the standard Sata interface.

You can buy any SATA II or III mechanical or SSD drive and it should work.

You can find exactly what type of drive you have by Going into the Apple Menu->About this Mac->System Report->Storage->Your Drive, and scroll down to Medium Type.

Any SATA 3 HDD/SSD will be compatible with your MBP. Here is a sample of what is available from OWC:

http://eshop.macsales.com/shop/hard-drives/2.5-Notebook/

There are other vendors as well.

Installing a larger capacity HDD than what was installed before is always a good option. The less that you must off load to an external HDD results in greater productivity and convenience.

If speed is important (time is money) then consider a SSD. There will be a significant reduction in response times but they are much more expensive than conventional HDDs.

Ciao.

Its always good to have a larger Hard drive if you can get one. If only because it will be able to store more stuff.

Other than that, an SSD will be slightly faster than a mechanical one in read/write operations so you may want to go that route, and you'll see an improvement in rendering and other things that read and write from the drive a lot.

You're welcome.