I understand about leaving baggage in coin lockers at Shinagawa station. But this is OP’s first time in Japan not familiar with way around Tokyo by train, hope people remember first time they were at Tokyo trying to figure way around.

There are nine coin lockers at JR Shinagawa station, all of them are inside of ticketed area. Only one coin locker in north side accepts Suica and all other coin lockers at JR Shinagawa station accept coins only.

This means OP likely need to get exact change where getting currency exchange in the U.S. departure will only provide OP with paper currency, no coins, so OP likely need to get some Japanese Yen cash at Narita before he boards Narita Express. There are three different size where largest one, 500 yen lockers, can accommodate typical size suite case.

However, again, remember first time visiting any country for the first time and struggling to figure out which coin is which? Also, JR Shinagawa station is divided to north side and south side ticketed concourse area. Middle is walkway connecting west and east side of the station which separate ticketed area of JR Shinagawa. If OP left luggage in south side coin locker and when came back to retrieve ended up at north side of coin locker… Remember, this will be very first time OP will be at JR Shinagawa station, let alone first time in Japan…

Again, I am emphasizing that this is very first time OP will be in Tokyo, under limited time, and likely OP does not read nor speak Japanese. I think these things are easy for seasoned travelers to Japan, but when we remember very first time we stepped out foot in Tokyo back when… I just think it will be best to keep things simple as possible for OP…