The problem with just doing stuff is that you don't know what they need! A lot of people tend to step up at the beginning, when they first get the news, but the family is still dealing with the shock and isn't in "help" mode yet. The other thing is, sometimes things aren't as hard for the teen (or adult) in the beginning and it's down the road, when chemo really kicks in, that they need more support.
Lotsa HelpingHands is a great (free) website and it allows volunteers to do what is needed with just one or two family friends coordinating it. So the teen's family doesn't have to answer all the well-meaning requests and coordinate all the influx of frozen meals. What you don't want to do is create more work for the family in coordinating all the volunteers! So you want to use a website that does the work for them.
They need more than meals in many cases. They need rides to activities for the other kids in the family, sometimes things like yard work and snow shoveling, errands, housecleaning (if they want someone in their house), and more. Maybe she needs DVDs and entertainment, maybe she needs a tutor to keep up with her studies (that can be worked out through the school), maybe they need someone to go to appointments with a notepad to write down all the stuff they're going to forget.
Moreover, if this girl cannot leave her home, there is concern about her immune system. So you absolutely don't want a huge influx of people into the home with food drop-offs.
The best thing you can do is talk to the family's closest friends - or if it's you, step up to be the coordinator yourself. Go to LotsaHelpingHands and familiarize yourself with the website and how it works. Then meet with the parents and explain it, and figure out ways to get the info to all their friends, the kid's school friends, etc. You take all the arrangements off the shoulders of the family, and you coordinate form a distance.
Their needs may change as they go through the treatment - so there may be 100 things they'll need in a few months that they haven't even thought of now.
The point is - they don't know. So don't just jump in and do something, anything, no matter how sincerely you want to help. If often creates more of a problem for the family, and that's the last thing you want to do.