Student seeking guidance

I've recently been diving into some philosophical questions, but I'm realizing more and more that I have a dull and untrained mind. I worry about the kinds of logical fallacies that I'm missing. I want to pick up a text that will give me a good introduction to formal logic. I'm not shy about textbooks (I'm doing my PhD in Biophyiscs or Bioinformatics), but I'm equally open to less rigorous texts. But I assumed that help from you lot will be better than me poking into this huge field unaided.

EDIT: Again, thanks for the comments! They've been very helpful. While I want to read everything, I think I'm going to start with "A concise introduction to Logic" based on upvotes here, ratings on Amazon, an apparent history of success (11 editions), and financial considerations. You all rock, and I look forward to having a blown mind. :)