For starters, smoking off a pop can is a fairly poor/wasteful way to consume your weed. My suggestion is that you and your friend buy, or get someone whose 18 to buy you a cheap pipe (I've seen pipes for 5 bucks), or an even cheaper (99 cents) pack of zig zag rolling papers. And If you can't roll, don't sweat it, theres about a bo-jillion websites that can give you instructions.

Good luck with the toking