Your next three tasks will take place one after another. Once you accept mission "Meet the Team" in your hideout, you should head to tunnel marked on your map. When you reach the new location, you should go to your new hideout and meet with your team. This task will be completed automatically after a short cut scene.
Use the laptop, accept main quest "Young Wine" and head to a small town marked on your map. Your task is to obtain two things: civilian clothes to disguise yourself and a wine truck that will grant you access to the wedding. Reach your destination and climb the rocks presented in the picture. There you can use your drone to scan the facility for useful information and enemies.
There will be several enemies in the area. First, take care of a guard standing by the gate. Although other soldiers are not likely to notice him, you may want to destroy the source of light above him. Right next to him, by the gate, there is another opponent. You should interrogate him as he can provide you with some useful pieces of information.
Approach the main building, located in the middle of the area, and wait for the guard presented in the screenshot to move behind the vehicle. It's a perfect opportunity to get rid of your opponent. Another soldier will be protecting the area on the right of the main building. Take him out with a headshot and don't worry, nobody will look for him.
Get into the house and use the stairs to reach the top floor. There is a guard on the right side that you can totally ignore. He is busy talking to a woman. In the neighboring room however, you will find the disguise you were looking for. Truck keys are located on the opposite side of the same floor. Before you actually leave the building, you can complete your optional task. You are supposed to eliminate a target located on the ground floor. A headshot should be enough, and the woman he was speaking with will not complain.
You can also secure another hostile facility - the Winery, before you proceed with your quest. There aren't too many guards left in the area, so it's wise to eliminate them before you move on. As soon as you are ready, enter the vehicle and head to your destination to begin Big Fat Wedding. The quest is nothing more than a cut scene, so all you need to do is get to the church and watch the ending of act I.