I know I'm repressed, and I've been trying lately to break out of my shell a little bit and "live a little" - to live in the moment without worrying about the future.

That's cool! But if I were you, I'd test those boundaries responsibly, with people that you trust, before I'd go bar hopping with a dude I don't know beyond one evening, when I'd never been drunk/had sex before. Make sure you know your tolerance, limits, and that you're comfortable saying no. Living a little doesn't mean living someone else's dream.

am I really going to regret it?

Only you know that. You might not. You might have three beers and the best night out of your life, culminating in sex that's easy fun with a lover who knows what he's doing, and knows you don't want anything serious. Or you might wake up the next morning, with no memory after tequila 5, no longer a virgin, with a creeping sickening realisation that he didn't use a condom, and that this is definitely not for you.

How do I know when I'm going so far that I'd regret what I'm doing?

I can't define it, but I know pornography when I see it, as the old judge said. Start imagining scenarios and your own reactions. Open yourself up to new things - including maybe going on a date with this dude, but keep a firm grasp on your own integrity and your right to say no and walk away, even if it makes you a "prude."

I think partying is great fun, and even poor decisions aren't the end of the world. It might even be worth going out with this guy, and being a little bit honest about not having a great deal of drinking and dating experience. Ask him to show you the sort of fun night he'd recommend, that's not going to leave you with regret in the morning. Challenge him, engage him, don't let him swamp you. Make it into a challenge to really wring the fun out of the night. If he reacts with boredom, distaste or an eyeroll, then he's boring as hell and wouldn't be worth the time anyway.