美國Selective Service System

This Q and A pretty much answered all of your concern.

"In the event of a mobilization, Selective Service Reserve Forces Officers would be called to active duty to establish State Headquarters and Area Offices at predetermined locations, and at the same time the Local and Appeal Boards would be activated. A lottery drawing would be conducted to determine the order in which men would be called, and induction orders would be issued, in lottery number order, by means of Western Union Mailgrams. The first priority group would consist of men in the calendar year of their 20th birthday. Registrants receiving induction orders would either report to the Military Entrance Processing Station for examination and possible immediate induction, or file a claim for postponement, deferment or exemption from military service. Such claims would be considered by the Area Office or the Local Board, depending on the nature of the claim. Agency mobilization plans are designed to meet the needs of the Department of Defense."

咁係咪姐係要去當軍尼~? technically yes, but chances are you will not be selected.
佢其實係咪抽人當軍嫁...會唔會好易抽中~? in your case, not really. People will high school education only, age about 20, physically well fit will be selected 1st.
同埋如果抽中左係咪一定要當,可唔可以揀唔去? If you fail the screening test, you don't have to go.

Everybody signed up and chances for you being drafted is very minimal. Cos US right now have so many volunteer military reserves.