what Ram do i use for Mac Mini

It depends upon what model Mini you have

Mac mini: How to remove or install memory - Apple Support

You can even replace the memory in earlier model Minis. See:

http://eshop.macsales.com/shop/mac-mini/?utm_source=google&utm_medium=cpc&utm_ca mpaign=sitelinks&gclid=COyxl4isscICFQET7AodNTcAQg

Make sure yo get the memory from a place where they say it will work in your model Mini, Macs are picky about memory. The link above is a good place to get memory. The also have videos for how to replace.

Go to the Crucial Web and it will tell you or scan your Mac for the correct one.