answering your questions as best I can (I'm a straight male too) in order:

1. if he/him seems right to you, then your pronouns are he/him. if other pronouns seem right to you, then your pronouns are those pronouns. pronouns don't have to match up with your gender or presentation, go with whatever you vibe with
2. when meeting new people, I give my name and pronouns. "hi, my name is salarua and my pronouns are he/him." of course, it's nice to give your pronouns when asked, but other than that it's up to you
3. just including your pronouns in your profile is good. some people put them in their nicks, some in their bio or about me. if you have a Mastodon, Akkoma, Misskey, or Firefish account you can put your pronouns in your custom fields
4. you can try and figure out other people's pronouns from how other people refer to them. many people will also give their pronouns if you introduce yourself with your pronouns. it's not a faux pas to not know someone's pronouns beforehand, although I admit I don't know a non-awkward way to ask someone their pronouns
5. a good bet is to refer to people whose pronouns you don't know as they/them. if you mispronoun someone by mistake, quietly correct yourself and continue with whatever you're saying. "so after arriving at the office, he- sorry, they went to go see their supervisor about the presentation..." as long as it's not done out of malice, people don't mind being mispronouned if you acknowledge the slip-up and move on
6. I haven't met anyone irl with neopronouns either. presumably people with neopronouns would go by them if they were among people they felt safe with. unfortunately most of the world isn't safe :(