As one of three new jobs available in the Skyward expansion - the others being Dark Knight and Astrologian - the Stagehand job in Final Fantasy XIV is like Bard's job in that it's meant to be played as a ranged DPS with support qualities, but offers a bit more flexibility for players who want to run it as a direct damage role. The Machinist wields guns, can set up turrets, and can run while using a large number of abilities, which makes the job very appealing to many players.
Before unlocking Machinist, you need to have completed a few tasks before unlocking the job: first, you need to have purchased and installed the Skyward and completed all main storyline quests in A Kingdom Reborn and have at least one job in level 50. Once these are completed, this will unlock access to the location called Foundation, located in Ishgard. Once teleported there, you'll want to find yourself in the Skysteel Manufactory and go to the NPC located at coordinates x:8.2, y:10.2: the person you want to talk to is called Stephanivien, who will give you the ability to start the quest So You Want To Be A Machinist.
Upon completion, you will gain access to Savior of Skysteel as a quest and upon completion of this quest, you will gain access to the Machinist's Soul and some basic equipment for the job, which will allow you to access the job. You should keep this NPC in mind as you will be coming back here to do work quests later.
While some may consider Machinist to be similar to Bard in having certain support abilities, those abilities are few and far between. This was more relevant when the position was first released with Skyward as for example your turrets were able to replenish MP and TP but this no longer applies. You have an ability with an AOE that increases movement speed and another that decreases damage taken by party members in range. You'll want to focus on dealing damage, and you'll do that in two different ways when you start leveling up: directly and indirectly.
Your main damage will be ranged and dealt through auto attacks and abilities: you'll want to focus, especially at first, on reassembly and wildfire abilities: the former increases the power of the next ability used and the second increases the potency of all weapon skills until the effect ends, resulting in a burst of damage. The other main method of dealing damage is indirect, through your turret and automaton: the first comes to you very early on and the other when you reach the current level cap of 80. Each only lasts for a limited time and has an ability overload that forces them to use a heavy attack before deactivating, so you should only use it when the turret is about to deactivate anyway.
Additionally, you should also note the Hypercharge ability, as it also unlocks other great abilities in your Machinist toolbox. As you spin, you'll earn points in your heat and battery gauges; The Heat Gauge gives access to Hypercharge and the Battery Gauge gives you access to your turrets.
This covers the basics you need to note to unlock the Machinist job and some of the things you need to know about the job to get the most out of it when you start. Hopefully this work will see some interesting additions when the Endwalker expansion is released later this year.