Asklemmy

First off, thanks for asking and wanting to be more inclusive! :)

For your pronouns, you decide. If you typically go by "he/him" you can keep doing that, it's up to you.

Depending on the setting, people around you might all introduce themselves with their prefered pronouns, you can introduce yourself with your pronouns if you wish to whenever though.

Similar to how you might tell people a different name you prefer to go by. So if your given name was Nicholas but you prefere to go by "Nick," you might introduce yourself like, "Nice to meet you, I'm Nicholas, but just call me Nick." likewise, you can say something like, "Nice to meet you, I'm Nick, He/Him"

Online, it's fairly common now to have your pronouns in your profile or bio. Again, it's up to you if you want to put them in your bio or not. Some sites have actual places in the sign up screen or profile page to place your pronouns, it depends on the site/software.

It's rare for people to get offended IRL if you unintentionally mis-gender them. Most people will correct you politely in the same way they might if you called them by an incorrect name. So if you said, "Hey Nicholas, how are your classes going this semester?" they might say, "Oh, you can just call me Nick. Classes are going well so far, how about yours?"

Often people that know them will correct you politely too if you don't know and used the wrong pronouns. I've had it happen a few times over the years and everybody has always been very polite about it. I just quickly say, "oh sorry, my bad." and then just make sure to remember their pronouns going forward.

I personally have some family and friends that use they/them vs she/her or he/him. It's a thing for sure, we all support them and their pronouns. It's not very tough to get used to, and as long as you correct yourself if you make a mistake, nobody will be hurt. It's fundamentally about loving them and being inclusive and supportive.

I hate you

Many other answers emphasizing to be on the considerate side are good. I just want to add two things.

Some folks have said that when the way you present matches your pronouns, you have less need to offer them. Personally, I think it is good to offer them anyways when meeting new people. People don't always present exactly in a way that you might expect their pronouns to indicate. As one example among many, someone may present very femme but prefer they/them pronouns.

This also helps us (you and me both!) unlearn some of our learned gender associations and habits of inferring gender based on appearances. It can sometimes be unpleasant to deal with someone that's clearly trying to figure out their gender identity visually or by voice, etc - trans or not. For an extreme example, there are even cis people getting harassed about which bathroom they're using (the one aligning with their gender) based on reactionary assumptions.

Finishing up that topic, offering your own pronouns is also a way of letting others know it's okay to be more open around you, that you are a bit safer than the average person. This can be most impactful, imo, for people who are trans or questioning but who aren't out yet. A lot of folks are struggling at that point in their lives and it can really help to know who is safer.

My final thought is that when you don't know someone's identity, it's good to get in the habit of using their name or they/them. If it's a real person irl, then you'd still want to ask for pronouns soon-ish. Occasionally, they/them-ing someone can also become unpleasant, though usually it'll be obvious from context (e.g. someone who is trans and strongly prefers he/him might perceive continued they/them to be a form of harassing them). Grabbing pronouns soon-ish avoids any awkwardness.