Here's a blog post from Jessica Gottlieb with some great job advice for Millenials, including:

• Your first job will probably be a crappy job. Don’t wait until you’re 20-something to have that first job. Working is good for you, even when it’s not fun. 
• No one owes you a job, you’re replaceable so it’s up to you to be better than everyone else (this advice holds true until you’re 75 or so)
• If you work in a service industry tips should feel like silver and gold raindrops. No one is obligated to give them to you. Customers can sniff out entitlement.

To this I'll add:

• Be on time.
• Watch your language. Not just cursing, but grammar. You don't necessarily need to know when to use "who" or "whom", but at least know that you shouldn't say "I'm doing good" when you're asked how you're doing.
• Dress appropriately and accept that it is perfectly reasonable for your boss to define what is appropriate.
• Use a firm handshake when you greet people, but don't try to break their knuckles. Also, unless someone's your friend don't give them a hug/bro-hug.
• This is for guys: shave every day even if you don't think you need it. Believe me, you do. If you have a beard, keep it trimmed and shave where appropriate every day.
• Flirt at your own risk. It can backfire on you in a big way.

There's plenty more where that came from; things I was once taught and that have never really changed.

Side note: you millenials are exactly where we Generation X/Y, Baby Boomers were at your age. The generations that preceded us thought we were slackers/losers/lost causes too.