By interacting with Sturges in Sanctuary, you'll have an excuse to learn Fallout 4's settlement gameplay. As we covered in our Beginner's Guide, building things isn't the most intuitive process. Start by going to the Workshop to enter Workshop mode. Move around the town to highlight and salvage fallen homes, leftover furniture, and other items for scrap. These will be the materials for your new structures.
Fulfill all of Sturges' settlement-building requests by building beds, water sources and a small farm to grow food. The materials for these items should come easy from salvaging the unwanted items in town. The last requested item will be a turret, the materials for which you might need to find in a couple missions outside of Sanctuary or through general exploration of the other towns and cities.
Don't hesitate to pick up everything, especially when it takes mere seconds to transport your haul to a settlement via fast travel. The requirements for fast travelling are simple: don’t exceed your carrying weight and the location has to be a place your previously visited. Take full advantage of fast travel so you can pick up and dump lots and lots of junk for scrap.