Small talk is stuff like "How're you doing?" "How's it going?". If you get asked , you just say "I'm good, How're you?"
It's like a script - no one is really wanting to know the answers to the questions, it's a kind of social glue.
Watch other people doing it, and learn.
In terms of eye contact and smiles, just keep it brief.
Avoiding eye contact is a definite way of telling people to leave you alone, so just catching someone's eye with a brief smile and a "hi" is enough to say you're open to a chat.
If I walked into a room of people, I'd probably do a quick glance round, a quick "hi" to everyone, and leave it at that.
Not everyone is a rampant extrovert. Extroverts are great though at creating space for quieter people to just be themselves because they hog the limelight, so make use of them.