My best friend and I are going to be traveling to St. Louis by plane. We are not old enough to rent a car to get around once we arrive. Anyone have any tips for travel that won't cost too much?

Get a fake ID?

Maybe buy a weekly pass for $27? http://www.metrostlouis.org/FaresPasses/FareChart.aspx

Did you take a look at Metro Transit - St. Louis's website?

Exactly how old are you and your friend? Hotels have minimum age requirements, too.

we aren't staying in a hotel. but we need transportation to get us to where we are staying.

Use Uber
https://www.uber.com/cities/st-louis

Uber??? Is it safe??????

I've never heard of uber but I'll look into it!

Yep. It's safe.

Load the uber app on your smartphone and get going.
If you don't have a smartphone, I'm not sure what your options are.

Let me play the bad guy here.
You are not getting a hotel room, you are too young to rent a car, you have no clue how to use a metro system, and have never heard of Uber... Maybe the best idea would be for whomever you are staying with to pick you up.
Have you ever flown by yourself before?
Please do more research. It's a cold, cruel world out there and I would hate to see you taken advantage of.

Zipcar rents to people 18 and up.

Wouldn't the people in the place you are staying provide info on the best way to get there?

And agree that you seem to be missing a whole lot of very basic info.

That's a great idea, jubilada!
http://www.zipcar.com/stlouis/find-cars

Are you running away? For some reason it feels like it - and I hope I am wrong.

Metrolink light rail has stations at Lambert (the airport). Perhaps there is a stop close enough to your destination, or someone can pick you up at the Metro stop.

http://www.metrostlouis.org/Default.aspx