It can oftentimes prove helpful to browse reference materials as a way of finding a topic for research that is of personal interest. One way to find reference material is to browse the Reference Collection in the 410's.
Another approach would be to search Library Search using the term Applied linguistics -- Research -- Methodology or Linguistics -- Research -- Methodology. Some recommended books include:
Projects in Linguistics: A Practical Guide to Researching Language
by
Alison Wray, Kate Trott, and Aileen Bloomer