The game opens up in the midst of a robbery and when you gain control of your character, move to the guard. Aim the gun at the people to make them move into the back room. Get used to the controls if you want right now, but there will be a better chance coming up.
Once all of the people are in the back, bring up your phone and contact list, then call the number to detonate the bomb. The vault door will now be open, so head to the back and go inside it. Feel free to pick up the loose cash, but the real score will be in the back.
As you're coming out of the vault, one of the guards grabs your character and you will need to switch to the other one nearby. Follow the prompts on the screen on how to switch characters and when you do, shoot the guard in the head. Follow the others and take cover behind the crates, waiting for the explosive to open the door.
Hit the shutter switch and you'll be confronted by some cops, so this is your chance to get a better grip on the controls if needed. Keep shooting the cops until you can move up, then fight the cops some more. You can't use the vehicles for now, but you can use them for cover.
You'll need to keep fighting down the road until you get to the getaway vehicle. Watch the cutscene and once you gain control again, you'll be driving the vehicle. You'll see a yellow line on your map and if you've never played GTA before, it's basically like a GPS and tells you the route to take to your mission destination.
Keep following the path until you get to another scene, you'll be in control again. Use the dumpster for cover and keep peeking out, taking shots at the cops when you can. Keep fighting until another scene plays out and brings you to the end of the mission.