Apple has a website that you can search. Or Google give you oodles of mac specific shops that can give you this information faster than asking the question here.
HERE is the manual for the 2011 mini. It explains not only how easy this is, but it also says exactly what specs the RAM is...on pages 41 - 46.
OWC (use google) has good inexpensive RAM. Others like RAM from other manufacturers or retailers. DO NOT purchase RAM from Apple, same quality product as anybody else, but you get to pay quite a bit more.