So I've been trying to ask different professors I've had and done well in the class for letters of recommendation. I've asked three so far via email and its been over a week and none of them have responded...so I take it as a no. So now I have no idea what to do and who to ask because I didn't really get to know any of my science professors.
And how can I get professors to be more receptive to writing me a good letter?
1) If it has only been a week, they may just not have an opportunity to decide or even read your email yet. For some, this was just a 3 day weekend. On top of that professors are busy.
2) Do they know you by name? They may need to take time to review your performance in the course before they offer to write you a LOR, and this may also contribute to the delay in response to you. Face to face requests can be helpful b/c they can jog the professor's memory when a name does not.
At some big schools, science classes are huge and you may not have the opportunity to have a small, intimate science class, especially if you didn't do more than the pre-reqs. If this is your case, professors will probably be used to writing LORs for students they couldn't pick out of a line up. The LOR will be very matter-of-fact and impersonal . . . but sufficient as long as you have other LORs that are from people who know you well. If that's the kind of LOR you'll need to settle for, you should also offer a portfolio of any impressive work you did in that class so they can make specific references to your work rather than just generalizations about the criteria for earning the grade you got. If you are not from one of the huge state schools where this class size and restriction issue happens, it is even less favorable to have one of these impersonal letters.
3) Did you use your professional email address (.edu account) or otherwise an account via which you had previously communicated with them? And had you done well in the course?
4) Professors typically politely decline rather than just give the silent treatment with regards to LOR requests. However, if you're at a school where there are some seemingly disinterested instructors, maybe they're also unprofessional in this way. Writing LORs and turning away requests you cannot support is part of the job description of a professor.
5) What else did you give them? Did you just send an email asking for a LOR? That's not proper LOR request etiquette. You should provide them with a packet, which may include some combo of the following (necessary items bolded):
CV (you can put a professional picture of yourself on your CV if you think it will help jog their memory),
Personal Statement (if not written, you can write directly to them about why you want to go into medicine), transcripts or written evaluations (at least for related coursework),
waiver of right to read the LOR (at least just in writing if not giving the official form until after they say "ok"), submission of any impressive coursework you would like to remind them of (research paper, A+ exam, etc.), MCAT score if available.
With regards to getting a LOR from a future professor:
Pay attention, ask questions, go to office hours, be engaged, do well in the course. You can let them know ahead of time that you are hoping to perform well enough for a LOR and ask what they expect/require.