Just go to the Digicel in Negril (same mall as Popeyes)...they'll set you up with a sim card including placing it in your phone for you. Bring your passport for ID and a sim card should cost around 700-800j.
Wait time is literally based on how many people are in there.
And, your phone will work like any phone based on the kind of plan you need...phone calls, texting, data etc.