Mission 1 is relatively straightforward.
After the sort of tutorial you are tasked with destroying 4 gun crews. To be honest you don't need your sniper rifle as most of it can be completed with the pistol and knife (Y up close).
The gun crews are mostly 2 men loading/firing and a guard watching. Note when the gun fires, the screen shows a noise symbol which means you can kill undetected (useful if your going for the undetected acheivement).
When you have killed the 4 gun crews a cutscene shows the level exit. Exit and cheivo will pop!