My dissertation thesis deals with linguistic benefits of the CLIL approach at a bilingual secondary school. I would like to do qualitative research but I'm not sure about which qualitative methods to use in this case.
No one can give you general answer telling x is the best. Telling which is suitable depend on what would help to answer your research question.
A good way to start with is to look at the literature and see what methods used and why. This will assist you to choose your research method and.justify.
Keep in mind, choosing the reseach method depends on what would help to answer your research question not what you like to do. With revewing literature you may end up not to choose qualitative approach, you may choose quantitative or mixed methods ... Literature is a kay for choosing.
Rayed is right. It depends on what are you focusing on. A colleague of mine and me ran a research on the use of multiligualism in two high schools. We used a mix research approach using focus groups as a qualitative approach (sure after the literature review was ready) and later on we developed a questionnaire and ran quantitative analysis.
However, really you have to define accurately your research objectives and focus.
I would like to suggest to read the recommendation of Francisco Donoso-Maluf to make a clear distinction on phenomenological traditions. He posted substantive answer about qualitative research.