Linguistics, as you may know, is a broad subject, and if you choose to study it you will have to gain a basic grounding which will see you cover the spectrum of the subject.
However, this broadness means that –especially if you want to enter a linguistics-specific field – you will have to choose a field in which you are particularly interested at some point of your academic career.
Some specific areas in which someone studying linguistics at university may specialize in include:
Applied linguistics: As the name suggests, applied linguistics refers to the use of linguistics to solve problems related to language. The initial focus of applied linguistics was the teaching of second languages, and helping people to communicate across linguistics boundaries still remains the backbone of the subject. It is, though, much wider in scope, and is known for a being a particularly multidisciplinary branch of linguistics.
Cognitive linguistics: Cognitive linguistics looks to explain the mental processes behind language. One of its central tenets is that linguistic knowledge is not different to other types of knowledge, as opposed to some other branches of linguistics which consider the linguistic portion of the brain as being distinct from other mental processes. Cognitive linguistics is a relatively new branch of linguistics.
Comparative linguistics: The goal of comparative linguistics is to compare and contrast different languages in order to identify similarities and differences between them, with the ultimate aim of finding a common root and systematically explaining the differences. This can sometimes involve the reconstruction of a dead common ancestor of two languages.
Semiotics: Semiotics is the study of signs (something that can be said to mean something), their meaning, and the processes that govern the relationship between the two. It is not necessarily limited to words, and can include anything that has some sort of cultural, societal or other type of significance – think about a green traffic light, or an offensive hand gesture for instance.
Lexicography: The compilation of a record of the words we use is the objective of lexicography As well as writing dictionaries, lexicographers seek to improve the methods that are used to do this by evaluating current compendiums of words. Theoretical lexicography also looks to analyze the vocabulary of a language, looking to investigate what links certain words in terms of their physical make up and their meaning.