Today I learned there are people affiliated to nazi groups in my town, and I'm scared as fuck.

Mentioned elsewhere, but reaching out to known safe people is a great place to start. The feminist collective and antifascist guy from ig sound promising.
I think there's two flavors of actions to take, and you can decide a balance that's right for you:
Don't be an easy target. Your online/public persona is ineffective lib #resistance - you're against Nazis, but Nazis won't see you as a threat. Embed yourself in some community (e.g. a shared hobby), be friendly and likable, and tolerate (or just lightly push back on) the reactionary bullshit that will inevitably come out of people's mouths. Your goal is to have a group people who know you and think fondly of you, who don't want bad things to happen to you specifically. If you're in a minority group, having some chuds think you're "one of the good ones" is tactically useful. There's obviously a balance with the emotional toll of being with chuds and hearing gross takes. You can probably at least play the "let's not talk about politics" card, but hopefully there's at least a group of #resist libs that won't spout too much garbage? Obviously the best option would be like minded people - maybe have a weekly meetup or something with your art friends?
Punch Nazis. They are dangerous, and their ideas needs to be clearly denounced and challenged head on. Remove stickers, or cover with your own. Spread rumors about the folks you know in the groups. I'm 50/50 on what someone mentioned about getting a newspaper involved - that might backfire by increasing their notoriety. They're in these groups because it makes them feel cool - make it clear that nobody thinks they are cool, and actually they're disgusting and pathetic.
Please don't feel like you need to be actively punching Nazis in order to be a good leftist. Nobody can tell you what the "right" balance is - some folks need to stay covert to survive, and few people are able to publicly advocate what they truly believe should be done about the rise of fascism (especially now).
The existing organizations in your area and your personal risk tolerance dictate what your course of action should be.

I see you’re asking about opsec, but you should send their names, anonymously, to anybody you can think of that will bring them to light imo.
Their parents doing something about it is a long shot, but maybe the school? Local civil rights place? Newspaper? Other local services, etc.
- There isn't any local civil rights place (not the us) and I doubt newspapers can put names out there just because they're members of a group. Even with violent criminals, they only put out name initials.

This person is in high school. I'd think the first question would be: who is someone over 25 you can trust, and what do they think about this situation. A situation like this is complex enough for an older person to think about, let alone a high schooler.

Hmm, what if you put up the stickers of several different orgs to make them think they're outnumbered?

This is quite scary, especially if you live in that country with The Fart in power.
What is also weird is if you went round talking about workers rights or acceptance, you would be deemed to be the threat, or brainwashed.
- I don't, it could be worse. I live in europe, with a socdem-left coalition gov but with a rising fash-rightoalition gov but with a rising fash-right.
  I think there have been demonstrations in defense of the welfare system, but in neighboring cities
  There's also a feminist collective too, now that I remember.
  
  I highly recommend reaching out to the feminist collective! They'll likely have local insight and advice that we don't have.

Usually local broad coalition can help combat Nazism when there is a lack of socialists.

Sorry to hear, comrade. This advice is absolutely a shot in the dark, what works for one doesn't always work for others.
Maybe try forming an org. Maybe carefully put stuff on social media, making every effort to stay anonymous. Also randomly posted stickers and flyers around very carefully. It can take years but you might find people like you with no place to turn. There might be a lot of frustrated people with no other place to turn. It just needs someone to get stuff started. Growth may be slow af in the beginning tho.
Obviously things may be very different for you in your town. And the risk is there. Having IRL people who's opinions align Hexbear is very good tho.
Other people can possibly comment on this and refine ideas. I'm half spitballing.
- I could try putting stuff around on social media, but I have no idea about forming orgs, and I don't think it would be taken very seriously either (I'm finishing high school)
  I did have a chance to make friends with people that I think were more or less based left, but I didn't use it because social anxiety. They're the artsy hs people I mentioned. Aside from them I guess I could get support from antifascists in neighboring bigger cities, but I don't know what else.
  Maybe I'm being overly pessimistic, the solid left party got like 700 votes last locals, which is almost nothing compared to the main parties (libleft, libright and fash) but is still a lot...
  I could post stuff denouncing the stickers on instagram, but they've been there for a long time and it could seem like it's been too late, or something like that, idk
  I think I'll do that, and if it actually gains traction I'll ask here about what to do
  
  You're right. Trust your gut.
  the solid left party got like 700 votes last locals
  I think that if you IRL knew 10+ of them, you'd be so much happier. Sorry that it's hard and scary to be a leftist. Really.