As soon as you exit Citra's Temple, this mission will start. You will need to investigate Badtown, which is located far to the northeast, but the good thing you can fast travel there right now. Do so, then activate the Radio Tower just north of the town. Go back to town and visit the bar, where you will need to play a game of poker (this one is just a cutscene).
You will now need to follow the man in white without him noticing. Go through the door and head outside, where you'll find the man just ahead of you. Any time you get him out of sight, you will have 20 seconds to get him in sight again. Make use of cover whenever he stops, making sure to not lose sight of him for too long. Once he goes inside of the building at the end, this mission will be finished.