It helps if you say which terminal.
There are SIM card retailers at the exit of Terminal 3
In T3, there will be kiosk from SMART and Globe Telecom just after you claim your luggage. Just approach them and see what plans they have to offer.
Globe and Smart are the 2 big players. IMO Globe is generally better for internet coverage and speed. Ask for an LTE SIM card. You then need to buy some "load" (credit) - usually with a scratch card. Then you need to text GOSURFxx where xx is the amount of cash to activate internet access.
Google GOSURF and globe for details.
At terminal 2 last time I was there, the GLOBE SIM card retailers were on the top level where departures are. So, if you are arriving at terminal 2, walk upstairs after you exit customs and buy the SIM card there.
At terminal 3, after baggage claim, just before the exit, there are kiosks for both SMART and GLOBE. They offer 'free' SIM cards but the minimum reload cards are at P 300. If you can wait and get to a 7-Eleven, you can get the SMART SIM for just P 40 which comes with 7 days FREE 30 Mb internet daily and accumulated to next day if you don't finish them... and 35 FREE SMS for 3 days, unless you reload before expire at the sari sari stall for a minimum of P 15 for 15 days extension. Even if you don't reload, the daily FREE internet is extended for 1 full week!
I'll be arriving to Terminal 3 at 1am... what time are the kiosks open until?
I have seen the Smart kiosk open at 11 pm - not sure if they are open after that.Edited: June 02, 2016, 3:57 pm
To answer my own question, the SMART and GLOBE kiosks at T3 are open 24/7. I got a 800mb/day for 7 day sim card for 300php. For 500php, you can get the same card but with calling minutes (incoming calls are always free for either card). Good deal!
If you're at T1, there will b multiple booths outside the exit. There are globe, smart etc.