The first levels of Hitman are very linear. You must perform actions in a predetermined way in order to learn the basics of the game (infiltration, disguises, remaining hidden, eliminating the mission target).
Once you reach the landing zone, enter the mountain base and watch the cutscene. Soon the first training will begin. Your objective is to murder Kalvin Ritter who can be found on the yacht. Walk towards the bridge that leads to the yacht. You will learn that only authorized personnel can use it. Go towards the cargo loading zone. You must sneak up on the mechanic, stun him and move his body to the toilet. Then use his clothes in order to obtain the Mechanic disguise. Now you can use the nearby platform (the one for the cars) and enter the yacht.
Carefully bypass the other mechanics that can be found on level 0 of the yacht (they can recognize Agent 47 despite his disguise). Walk upstairs to level 1 and use the instinct suggested by the game to locate Ritter. He should be somewhere near the bar. Follow him. You will be stopped before entering level 2 - only crew members can walk through that part of the boat.
Go to the kitchen on the first level of the yacht. Sneak up on the crew member, stun him, hide his body in the large fridge and put the yacht crew disguise on. Now you can use any stairs that lead to level 2. Find Ritter again (don't let him see you or he will recognize you). Position yourself at the nearby bar and press the button responsible for hiding in the crowd. When you're hidden among the crowd, you don't have to worry about being detected, but you can't perform any additional actions as well.
Wait for Ritter to meet with Mister Norfolk. They will go to Ritter's quarters. Follow them. A guard is standing at the entrance to the quarter, he won't let you in. Find the open side window and use it to get inside. Position yourself at the wall, wait for Norfolk to turn around and select the handgun with suppressor from the inventory (ICA19). Shoot Ritter down. Once the assassination is completed, leave the quarter by using the window again.
Now you must leave the yacht. Don't use the main bridge, you would be recognized by crew members that are standing there. Instead of that, go back to level 0 and use the car platform again. Go back to the starting location and interact with the marked vehicle.