Books and reference sources tend to be the most useful sources for literary criticism. Books of criticism may focus on an author's entire work or on a specific work and will usually be more in-depth. Reference sources usually give an overview of an author and then often cover specific works. If you're not familiar with an author or a title, reference is a good place to start.
Articles tend to cover very specific aspects of criticism, so you might want to go to them last, after you have an overview from reference sources.
Not sure where to start? Try the guide to ACC library literary criticism resources.
ACC Libraries have more than 90 journal/magazine article databases. Here, I list the most useful ones for research on English Language Arts. (Or see a complete list of ACC Library Databases)