Hello Preety,
You can get some ideas as how to prepare for this please visit this link
USMLE Map
You can apply anytime during your medical school, since you are in the final yr, the best would be to complete your final and internship. Then you can sit for USMLE preparation in the mean time you can keep visit USMLE section in the xenoMED and ask any kind of question other member will help you to get the info you required.
And you can read more about in its official website Click Here
USMLE
Best of Luck