how do you get into paganism, satanism, witchcraft, etc?

I would first ask yourself why it is that you feel a draw to alternative forms of religion which you have no prior connection to. If you don't consider yourself a really religious person i feel like you would have difficulty adhering to an in depth spiritual practice with real obligations. May be something more similar to Chaos Magick could be more up your alley? 

There is no instructor you can go to or a building that can set you on a spiritual path, at least not one like I believe you're hinting towards. There are plenty of resources online at the tip of your fingers though! Countless YouTube channels make content on esoteric practices and spirituality. You need to ask yourself what you're looking for, is it community? a feeling of control or power over yourself? do you want to do magic operations or focus more on devotional work with a chosen spirit or deity? A spiritual practice could be daily meditation, quiet prayer in your head, drawing seals and sigils on the inside of your shoes or phone case. There are endless possibilities!!

I would also definitely recommend listening to the Glitch Bottle podcast, it leans more towards the western esoteric category but there's episodes on Santa Meurte, Esoteric Catholicism, and Paganism.

Along with that I would recommend the book Consorting With Spirits by Jason Miller. It isnt designed to be your first book, but it gives fantastic guides for basic rituals and prayers to a variety of spirits and deities pagan and Christian.

Oh also, yes you can believe in whatever you want to believe in, while some systems of belief may have stipulations about the worship of other dieties its up to you to find your own justification for your induvidual practice

Paganism, satanism, and witchcraft, are separate things (especially Satanism). I know that a lot of people use paganism and witchcraft interchangeably, but I've always felt paganism should be used to refer to the ancient pre-Christian religions. There's many of those out there and most of them have some kind of revival or reconstructionist movement, (for example Hellenism, Heathenry, Kemetism, etc.) if any of those interest you, there's loads of resources online you can use to learn about them.

As for witchcraft, pretty much all modern occult, magick, or witchcraft practices, including Wicca and chaos magick, can all be traced back to Aleister Crowley. I should note that I am a fan of Crowley and I consider myself a Thelemite, so I'm not entirely unbiased here, but if you ask me, Crowley is one of the best ways to get into that sort of thing. Crowley provides a good foundation for exploring the world of witchcraft, magick, etc.

Magic and occult knowledge isn't for "curious people", but for those who NEEDS knowledge and wants to understand the universe.

Otherwise, you'll be drawn into some crazy sects and this kind of  shit.

But if u really want to understand the universal laws behind supernatural, I would recommend u reading "Dogme et Rituel de la Haute Magie", however it is called in english.

Then, after reading it, you'll be able to discover how to actually work with pagan gods, witchcraft and all of this. There's many occultists that devoted themselves to work with and study paganism.