I am a straight male

Being straight doesn't impact what your pronouns are! Your gender, whether you are cis, non-binary, trans, or any other gender identity is what determines these pronouns.

are my pronouns he/him?

Probably but thats up to you. Usually pronouns are given in that order as a matter of standardisation for understanding where they go in a sentence. Ie "He is going to the store" or "it all comes down his choice." Obviously this leaves out some other gendered standard pronouns like formal Sirs and Ma'ams but for most conversation knowing she/her or they/them gets you most of the way there. For standard ones it might be intuitive but for people with neo-pronouns it helps a lot.

do you tell people when you meet them?

Sometimes. I try to do it more these days as a matter of course but usually I will give them when I'm in an environment where some people do and don't where no one does. They are in my email signatures and real social media bios though.

Online rules

At least on your profile somewhere. I like Hexbear's "no exceptions they will be in your name" policy, its better for uptake and helps people feel included.

figuring out others

If it isn't clear you could always ask. If you aren't a dick about it then it's usually fine. If you get corrected though the most respectful thing you could do is just say sorry and use the right ones from that point on.

non conventional pronouns

Yes people do use them, if you are in places that queer people go to or feel comfortable you are more likely to meet someone with them. I would argue its more polite to refer to them as neo pronouns but I also have boring ones so its not really my place to say. If you look around hexbear there are a couple of frequent posters with neo pronouns and I've known a few people irl to use neo pronouns as well.