The First Step

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

Detailed walkthrough
Upon returning to Sanctuary Hills at the end of When Freedom Calls, Preston Garvey will approach the Sole Survivor and ask them to help rebuild the Minutemen. After the Sole Survivor agrees, Preston will tell them about a nearby settlement that has been having trouble with raiders. Preston will ask the Sole Survivor to travel to the settlement and help them out, hoping that the settlers will choose to support the Minutemen in return.

To proceed with the quest the Sole Survivor must travel to the settlement and talk to their leader, who will give the details: a large group of raiders has been harassing the settlement and causing trouble. The settler will point the Sole Survivor to the raider hideout and ask that they are eliminated for the safety of the settlement.

To complete the objective, the Sole Survivor must travel to the raider hideout and eliminate the marked raider. Unmarked enemies do not need to be killed to complete the quest. With the threat neutralized, the Sole Survivor can talk to the settler from before to receive thanks and a reward of ~100 caps, as well as ownership of the settlement (and thus access to its workshop).

To complete the quest the Sole Survivor must return to Sanctuary and talk to Preston Garvey. He will congratulate the Sole Survivor on their success before presenting the rest of the reward: a flare gun and several flares that can be used to summon a team of Minutemen to the Sole Survivor's location. As the quest completes, Preston tells the Sole Survivor more about the fall of the Minutemen. He expresses his hope for the future before offering the Sole Survivor 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; the Sole Survivor can return at any time to accept his offer. In either case, Preston will begin offering Minutemen radiant quests to the Sole Survivor in order to recruit more settlements.

In order to continue the Minutemen questline, the Sole Survivor must agree to become the General and continue recruiting settlements. After recruiting at least four settlements, Preston will offer Taking Independence, initiating the next phase of the questline.

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.

Bugs

 * In certain locations, NPCs might spawn in inaccessible locations (for instance, Roger Warwick at Warwick homestead might spawn inside the water cistern). This bug is shared with the other settlement quests.
 * This can be resolved by using the console command to teleport the NPC in question.
 * The Warwick homestead example can be resolved by building stairs into the cistern and a bell outside of it. Using the bell and waiting for an in-game hour (using the Wait option) will cause the NPCs to spawn near the bell.