where is my os on this I have a new wireless modum from at&t/new kaser net pc/ dell 18 inch monitor all hooked up

I have a new wireless network modum from at&t, anew kaser net pc, and a older monitor 18 inch from dell have apple so i guess i have mac trying to find os can't I go to apple icon ,it says to choose about this mac that does not show on my computer what do i do? can somebody please help?from pam135