Will you write me a letter of recommendation? 
I typically only write letters of recommendation for students that I have taught in two or more coursesit makes for a much stronger letter if I can speak to your strengths and achievements across these multiple contexts, especially for letters accompanying graduate school applications. If I agree to write a letter for you, please follow the guidelines in Leonard Cassuto's excellent piece on how to ask for a letter of recommendation (PDF).

What should I know before applying to a PhD program? 
William Pannapacker, a professor of English at Hope College, has written at length about the state of doctoral education in the humanities. Read what he has to say here, here, and here. Then read what economist Dave Colander has to say about the job market for English PhDs, which discusses trends that are relevant to disciplines across the humanities and social sciences: the article is here and there's a nice writeup of the article's main points here.

Because a PhD is ultimately a path to employment, you should be aware of job placement rates in your field: the MLA has numbers on this for humanities disciplines PhDs here, NSF has numbers for different disciplines here, and the Atlantic pulled together numbers from various sources here. Related to this, before applying for a PhD you should also see what kinds of jobs are available and what their listed qualifications look like: for linguists like me, that means looking at the job postings on the Linguist List, the LSA's Career Center, and the Academic Jobs Wiki, but most professional organizations offer similar resources (e.g., the MLA, NCA, ASAAAA).

You should also know that there are significantly more people getting PhDs than there are academic jobs: the numbers vary by field, but you can see an example of this trend in science and engineering hereIf you are interested in a job teaching and researching at a university, you should read historian Erin Bartram's very frank discussion of her decision to leave academia after years of (unsuccessfully) trying to get a permanent university position, available on her website here. You will also want to learn more about the so-called "adjunct crisis" in higher education: read what the AAUP has to say here as a starting point. Finally, if you are serious about applying for a PhD, you should also know about mental health issues in academia: two reports on mental health issues for graduate students can be found here and here.

Doing research