How Competitive?

There are several posts similar to yours, plus a very informative post (FAQ i believe) in the Radiology section of SDN.

Nothing is guaranteed for matching into any specialty with regards to Step 1 scores, but my dean told me that a step 1 score 1 s.d. above the mean (i believe that's a 237) would be competitive for radiology.

My advice would be to obviously do well in clerkships, get strong letters, and consider doing research/finding contacts in radiology at your school.
Also prepare for Step 2 and plan to take it early and do well.
A meeting with your dean or program director at your med school might be helpful as well (maybe after you do a couple of clerkships, to show that you have a strong clinical knowledge base). But concentrate first on doing well in your clerkships, and keep an open mind about all specialties.