I am 15 and want to travel to Germany alone but am afraid I would not be able to because of my age. Is there something I could bring to the airport that would allow me to be able to fly to Germany alone from the USA? Or do I have nothing to worry about? I would be staying at a friends house and would only be in Germany for two weeks.
A letter of permission from your parents and/or legal guardians and them buying the tickets for you.
At 15 most airlines will transport you as long as you can show your parents allow you to travel, you will not need to travel as an unaccompanied minor, although if your parent want you to I think you still can.
Other methods of travel often have different ages for permission but USA to Germany will be flights. It will be best to have a direct flight (so not stop overs or even change of planes) between where your parents can drop you off and where adults who will be responsible for you during your stay in Germany can pick you up at the airport. But as the USA is a big country you may have to have a stop in the USA, if so, talk it over with the airline.
Self connections should be out, as you will be a tad young to be on your own in an airport. (More so in the USA than in Europe, due to different laws in the different areas.)
Without your parents/legal guardians permission and help you will not be able to make this trip, airlines and immigration officers will likely stop you without those letters and help.
There are many stories of people who (used to) travel USA to Europe alone without permission, but you can not be sure you will slip through the net and 15 is too young to even try.
Legally, starting at age 15, you can travel alone on most airlines, also multiple legs and internationally, without any additional documentation (https://www.transportation.gov/sites/dot.dev/files/docs/Kids_Fly_Alone.pdf):
... at age 15 most airlines will allow a child to fly alone on domestic flights without any unaccompanied-minor procedures. Some airlines permit this for kids as young as 12. When a child has reached this minimum age for traveling alone without unaccompanied-minor procedures, the airline does not require evidence of parental permission to travel alone. If the child has a passport, he or she can even travel internationally.
Some airlines might have different rules, so check on your airline's website.
Some countries you travel to (or through) might have different rules too, so it would be a good idea to bring a written permission from your parents - but it's not required.