Second? I'm Quad-lingual and I'm 21.
I learned them in elementary and primary. They are:
English (Formally official and compulsory),
French (Basic Level-2 years. Intermediate level-4 years Advanced Level-5 years), Hindi(I gave up mid-way, 3 years) and Mauritian Creole (mother-tongue)
It wasn't that hard to start learning 3 languages at once at the age of 5...
But the best ways to learn a second language are:
Media. Watch movies (with subs) or listen to German songs. (I love Rammstein), read german mags. You can also watch theater plays. (The Magic Flute is my fav one). It will make learning easier
Pay for courses. Usually, Schools/private tutors offers a large selection of new languages to learn and are professionally trained.
Get a teacher that is super-strict. You will learn faster.
Reading and Translation. You need to learn the German alphabet an how to pronounce them. Never use google translate. Instead, buy and read books that offers both languages (like one side is in English and the other in German) or use Translator apps.
Suit yourself Converse using basic and common words. Weebs usually do that. It will be good if you have German friends.
As Eight said above:
being in an area where the language you want to learn is normally spoken is the best way to learn it
I learned Hindi the same way as I live in a Hindu locality.(Yeye, we have a lot of T-series people here). I then learned the basics and the alphabets when I was in primary school.
Don't worry about the time I took. It will take you less time, because you only need to know the basics as you are a teen who understands a lot of things.