It seems that human activities give rise to gatherings. There are lawyers' conventions and magicians' conventions. There is an element of shared
investigation; one wants to see if anyone has learned what to do with stray feathers when making parrots disappear.
There is an element of commerce; one can get the latest rabbit door for a top hat. And there is an element of hobnobbing with one's fellow wizards.
Science fiction conventions have been held for 70 years. Some are attended by a hundred people; the annual World Science Fiction Convention ("Worldcon") draws five or seven thousand; professional or fan writers and other artists, editors and just plain readers, the ignorant and the knowing, the good, the bad, the ugly, all rubbing elbows and getting in one another's way. You will probably find an Art Show, with a mixture of professional and amateur art, some for sale, usually by posting bids; a program of panel discussions - at a recent big con, among other topics, "Technological Opportunism in the 21st Century", "How Do You Extradite A Time Traveler?", and "Alien Sex Toys"; Guests of Honor, who may give speeches; a volunteer desk where you can offer to help out; a Dealers Room, with books, posters, model space ships, and magic wands; a costume competition, usually called a Masquerade - some people also wear science fiction and fantasy costumes for walking around the halls; a Fanzine Lounge, devoted to amateur publications in the s-f community and those who love them; a Program Book, which may try to explain things; films and DVD's, readings, autograph sessions, juggling, belly dancing, English Regency ballroom dancing, fantasy role-playing games, singing, a Hospitality Suite provided by the con committee, campaigning for the sites of future cons, and a maze of parties in people's rooms, some open to everyone, some invitation only, some strange.
You are welcome to participate. Indulge your sense of wonder. Look for what might interest you; the people you meet there are probably interested in it too. Whimsy is generally better loved than earnestness; in the endless fannish slang (which it is well to use sparingly). "sercon", for "serious and constructive", is rarely a compliment. Such attitudes have consequences, which we pretend we can ignore. Most of us are not much for social graces, which means we are easy on small mistakes, and hard on big ones, or is it the other way around? Keep your wits about you, and never lead with your right. Good luck.