You need to either recruit them or clear them out. You can only start settlements at predetermined places. The most obvious one is Sanctuary, where the Minutemen hang out. If you follow them there and keep helping them out they will tell you the basic needs for the settlement to grow, such as food, water, beds and protection, and then you just build accordingly. If you want to attract more settlers once you've got going you just build a signal beacon and hook it up to some power. People will show up randomly after that (at which point you need to assign them to jobs).
