Currently it’s unknown what the first mission of GTA 5 will entail. There are numerous fake videos out on the web claiming to show what the first mission is like, but not a single one of them are real. Rockstar would not leak such a valuable piece of information of GTA 5.
A lot is known about missions in general however; for example, there will be three main characters and players will be able to switch freely between them during the main story mode. There will also be missions where it’s crucial to switch between them as all three of them will be working on the same heist or mission.
As far as the details go, fans will just have to wait until the official release. It’s not until then that we’ll really be able to know what the missions are like in GTA 5, including mission 1. Once the game is released surely the first mission will be posted on YouTube within 30 minutes.
Check out the "suggested by the author" links below for more GTA 5 information.