I'd suggest you study the overview paper by Pang and Lee (2008), "Opinion mining and sentiment analysis": http://www.cs.cornell.edu/home/llee/omsa/omsa.pdf
It will give a good overview of the field, and if you wanted to learn about the most recent research, have a look at conference proceedings for ACL, EMNLP, COLING on the ACL Anthology website: http://aclweb.org/anthology/