Depends on what kind of Linguistics you want to do. Minimally, you will want to be familiar with Syntax, Phonology, Morhpology, Phonetics, and Semantics/Pragmatics. Most of the other fields (Historical, Discourse Analysis, Socio-, Acquisition, etc.) will utilize these basic concepts.
Also, find out what kind of approach to Linguistics you want to take. There are some very different philosophies of language out there and many departments only have one.