You will need to fill out and submit an application available on US's IRB webpage.
Broadly, you should think through your project in each of these aspects, to prepare for your application:
After you apply, the Initial Reviewer will assign a category to your application::
Exempt (no foreseeable risk)
Expedited Review (minimal risk)
Full Board Review (more than minimal risk and protected subjects).
Most research projects will likely be Exempt, in which case it requires no further review. Expedited Review projects are forwarded to the Expedited Reviewer. Full Board Review projects require the most time, with the IRB as a whole performing the review.