One of two things, or a combination of both (the second because its fun)
(a) Work quietly and discretely in the background to subtly inspire various people around the world to pursue research and policies that will benefit humanity, attempting to steer the human race to a ‘Culture’ like materialist utopia (as seen in the sci-fi novels of Iain Banks I would try to maintain free will as much as possible, only intervening directly in case of an imminent existential threat.
I would create some form of afterlife probably combined with reincarnation, but as a reward and learning experience only, I’m not into the whole punishment thing. I would make death a subjectively pleasant experience for those undergoing it though it would seem just as unpleasant for outside observers, I don’t want to encourage people to start topping themselves.
Basically I would try to be a benign godlike entity.
Or
(b) I send a valid launch message to every ballistic missile submarine and silo around the world but nullify the warheads before impact, I would also teleport every warship from the smallest to the largest of every naval power to a small landlocked lake.
I would then announce my presence by simultaneously broadcasting to every television, monitor, mobile phone or other device around the world and in the appropriate local language. I would explain to humanity that I’m the one who carried out the acts above, to prove my power, and that they need to shape up and start working together for the common good. I would announce that I will only intervene directly in limited circumstances but if anyone wants to chat or needs advice they just have to ask for me through any of the aformentioned electronic devices. I would naturally have to create some cool, awe-inspiring, or just different persona for myself.
I would then sprinkle a few dimensional Gates across the world leading to the various inhabitable planets I have just created spread across the universe so humanity can express its desire to explore and expand. Some filled with other intelligent species or life-forms, others a tabula rasa for the human race to shape to its own satisfaction. Some of these Gates would have other effects as well, none fatal or unpleasant, which could only be determined by some brave person volunteering to go through them.
Basic rules would be to play nice with each other, including the other species, apart from that its up to you. Just don’t break the planet, thanks.