Hi,
Shinjuku is an area but thre JR station Shinjuku is the busiest station in the world ( 3million people pass through it daily) with 200 exits. It is an experience ( you have not experienced tokyo until visiting shinjuku station)
No marriotts close by but a lot of reasonable hotels ( inc a hilton)
It can take a little while to get used to it but if you avoid peak hours its not too bad ( i used to use the yamanote line then the west exit to walk to the hilton). Several metro stations also at the station or closeby
It helps if before arriving at the station, you have an idea of the train line/metro to use so you can follow the bilingual signs)

Regards

TBS