Log off, touch some grass. Put yourself out there. Once you get a few people around you, it's relatively easy to leaver yourself into other friends groups.
> But I've heard everybody talking about Halloween costumes in the past few days and realized I don't need one cause I'll be sitting inside alone on Halloween. I'd like to start fixing this but don't know how.
Just say yes to the next offer you get, you'll only make friends if you expose yourself to other humans. HN is unlikely to be the place where you make real friends.
Specifically about the parties: You can invite yourself easily enough to things - just get them talking about it (i.e. "oh I hear you're having a Halloween party, anything cool planned?") - if you sound interested and talk for a while most people will invite you.
The only way to get good at social interactions is to keep putting yourself in social settings.