Fastest way to learn a new language?

Here ya are! Three sites that will teach you a language:
Duolingo
https://www.pimsleur.com/
https://www.babbel.com/foreign-languages

Technically the fastest way to learn a language is total immersion.

DONT EVEN TOUCH DUOLINGO OMG HE MURDERED EVERYONE I KNOW BC I MISSED ONE DAY OF SPANISH

Well, the fastest way to learn anything is to be motivated to learn it. Be interested in it - today you have so many ways to get to the goal - you don't need to buy a book, a disc or whatever, you have your computer.
Then, still, try to learn the necessary vocabulary first (of course with the grammar at your side). Try to work under real pressure - as if you were going to speak that language alone. And finally... treat yourself to a teacher or even better, a vacation in a country where people speak that language.

Go and live in the country that speaks that language. I am eeeengleesh and only speak English and if you don't understand I will speak loudly and slowly with signs until you do xxx ha ha

Hire an interpreter.

invent a new language

@A_Dragon_Riding_King said in #2:

Here ya are! Three sites that will teach you a language:
Duolingo
www.pimsleur.com/
www.babbel.com/foreign-languages
CHECK OUT DUOLONGO BRO

Forget the old one