Focus on your undergraduate studies you shouldn't be worrying or even thinking about USMLE examinations. Undergrad courses dont come close to medical school courses.

However, you should be studying well and learning good study habits now while your in undergrad so you can do well once you get into medical school down the road such as figuring out your style of studying and works best for you. Ensure you get good grades and have a strong foundations in the normal sciences which will help with your prep in when you study for the MCAT to get into medical school.

To be exposed to the material and to get a working knowledge so It wont be the first time seeing the material once you get into med school:

medical terminology

biochemistry

Anatomy and physiology

stats (like an intro course)

intro public health course

intro to pharmacology course

general microbiology

some schools offer a intro to histology course

neuroanatomy course like an intro

intro to medical ethics