I would get letters of rec from profs who you really think will write you a good one. A good way to go about this is ask upper class men how certain profs write. Usually they might be able to give you a good idea of who writes a good rec or not.
It is not necessarily you need a 4.0 in a class, but you need to be an active student. By that I mean, attend office hours, email profs, raise your hand and ask questions, and do good on the exams. This will illustrate that you are active and are interested in this class. Once the grades come back and you do decently and if you really enjoyed the class email or contact the professor (usually in person is the best way). Be sincere and let them know that you enjoyed the class, tell them what you liked about it and compliment there teaching style (honestly).
Tell them what you are pursuing and let them know that getting a letter from them would be an asset for you. Most likely they will be flattered and then say yes.
Usually, in order to write a good rec they need some information, for example a resume, transcript, a statement of purpose (why dental school), sometimes tests you took in the class (if essay). Also, you might want to take some time to meet with them and talk about your passions. This will give a prof a deeper understanding of your goals and passions, and they can get to know you a little bit and develop a covalent bond or something. That way, they won't just bull**** the letter and actually spend some time, since you gave them some time.
I hope this helps,
Regards,
DesiDentist
