The First Step

The First Step is a Minutemen main quest and achievement/trophy in Fallout 4.

Detailed walkthrough
This quest begins directly after When Freedom Calls, after talking to Preston Garvey in Sanctuary. Preston informs the Sole Survivor that in order to rebuild the Minutemen, they need to regain the support of the local settlements. He then points the Sole Survivor towards a settlement he heard is in need of assistance, hoping that the settlers will give their support if you help them. The settlement is randomly chosen from a list of nearby settlements.

At this point the you must travel to the settlement and talk to the marked settler there. The settler will inform the you of a raider group nearby that is causing the settlement trouble. To solve the settler's problem, track down the marked Raiders and eliminate them. (Unmarked enemies do not need to be killed to complete the quest).

After eliminating the marked enemies, return to the settlement and report your success to the settler from before. The settler will express their gratitude and reward you with 100 caps after pledging their allegiance to the Minutemen. The settlement is now unlocked and the workshop is available for you to use.

To complete the quest, return to Sanctuary and talk to Preston Garvey. He will congratulate you on your success before presenting the rest of your reward: a flare gun and several flares that can be used to summon a team of Minutemen to your location. The quest completes, and Preston tells you about the fall of the Minutemen before offering you the position of General of the Minutemen.

If the Sole Survivor accepts the offer, Preston becomes available as a companion. If they refuse, Preston remains in Sanctuary, and the player can return at any time to accept his offer and become the General. In either case, Preston will begin offering Minutemen radiant quests to the player at random, though they must accept Preston's offer before continuing the Minutemen questline regardless of how many settlements are unlocked.

Companion reactions
Since Preston is always at Sanctuary until the dialogue is completed (at which point he becomes a companion and can be relocated), all other available companions can be dismissed to Sanctuary to witness the dialogue, allowing multiple companions to have their affinities raised or lowered at once.