This is actually very easy if you know what to do. Almost any cell phone case store will sell SIM cards for < $5. Just say "SIM card" and point to the side of your phone. If they say "no", say "donde es?". Usually there will be another shop nearby.
I've bought them in Medellín at a cell phone store on Calle 10 just West of Parque Poblado and in the basement of the mall -- there's a cell phone repair place in the parking garage next to the escalators.
After you get a SIM, you'll need to put minutes on it. Go to any GANA window in the city. They're all over the place. Say "recargo" and show them your phone number. She will give you options for what you want to add. Just do something like "3 GB" and point to it. Don't just put $$ into your account here because you won't have the app or data needed to apply it to your data or minutes. Get actual data. Should be like 30 mil pesos. ($10)
If you're staying longer, go to TIGO in the mall but take your passport. They will give you an official SIM (the others are actually black market), but it has to be registered to your name.
In both cases, once you have a SIM, you can take your phone to any TIGO store and use the self-serve kiosk to put money into your TIGO account. Then you can use your balance on the TIGO Shop mobile app to buy more minutes or more data. I usually put $50mil in if I'm by the mall and see the balance is low just in case.
Ok, after typing that out, I realized it's not so easy. But I pulled it off effortlessly when within the first hour or two of landing in Medellín the first time I visited. It sounds harder than it is. Hope that helps!