To an adult language learner, the first language is always the hardest. It makes a lot of sense to pick an easy language from the start. That’s absolutely fine if you’re interested in learning, say, Spanish. But maybe you don’t wanna study Spanish for a year. Maybe you want to learn Japanese, Russian, or Mandarin. A year of Spanish wouldn’t appeal to you, and you’d risk burning out on the whole language-learning thing altogether. On the other hand, jumping straight into studying a hard language, with no other language-learning experience, isn’t wise either. So what can you do? Fortunately, there’s Esperanto. Esperanto is an artificial language deliberately designed to be very easy to learn. You can use it to lose your “language virgin” card, without having to invest years of time into it. In fact, a month or two will be plenty of time to get the benefits of studying a foreign language.
I won’t try to BS you. The Esperanto itself will be useless. As soon as I got the benefits from studying it, I data dumped it. The only time I ever encountered it in “real life” was when the King Of All Cosmos spoke it once in We Love Katamari 
However, the same thing goes for weights at the gym. Lifting a particular dumbbell is useless. You don’t do it because there’s an urgent need to rearrange dumbbells, you do it because it trains your muscles. Esperanto trains the multilingual part of you, the multilingual muscles of your brain. And it doesn’t take years to accomplish that. You could get the benefit within weeks, if you really devoted yourself.
Within a few weeks of learning my first leksiko vorto, I was able to shift my brain out of English Mode. As a native English speaker, my mind had been frozen in perpetual English Mode for the past couple decades. Even when I was studying freshman Spanish in community college, I was thinking about Spanish in English, the inner dialogue inside my head was Anglo. Suddenly, I was set free of all that, and I could shift my mind into “Esperanto Mode”, as though I was a native speaker. And I can’t explain how happy that made me. You just have to experience it for yourself. You could experience it with “Japanese Mode” or “Russian Mode”, but it would take years of difficult study. Zamenhof’s artificial tongue will put your mind “in orbit” very quickly, because it’s such a logical, regular, easy language.
When you switch your inner dialogue to another language– and it doesn’t really matter which language that is– it’s almost like going into a mystical trance. I believe that merely spending time in this altered state of mind will flex your linguistic muscles and prepare you for harder languages. It annihilates psychological barriers, self-beliefs like “I can only speak English” or “Adults can’t learn a second language”. It alters your very self-image from “I am monolingual” to “I am bilingual”, which new self-image gives you an incredible confidence boost when you go on to study Polish, Farsi, Korean, or whatever else your heart desires.
How do you teach yourself Esperanto? There isn’t really all that much to teach. The grammar is extremely easy and intuitive, and you can master it in a week, easily. After that it’s just vocabulary, and you’ll notice that almost all of the vocabulary “makes sense”. That’s because all the root words were carefully chosen to be cognates with as many languages as possible. In a certain sense, the Esperanto vocabulary is actually very small. It’s not that there aren’t many words– there are plenty. It’s that words are built up from root words, in a similar way to English root words but much more systematically, logically, and consistently. There’s something like 2,000 basic roots, from which the entire lexicon is generated by agglutination. Most real-world languages, 2,000 vocabulary items is a drop in the bucket!
There are lessons available for free, and a whole Esperanto language-learning community, over at lernu.net. The community is so intense about trying to spread their language, when you register there, they’ll actually assign you a human, volunteer guide, who will correspond with you over email. All for absolutely free… it’s amazing. If you have money to spend, buy yourself an Esperanto textbook, such as Teach Yourself Esperanto
Esperantists dream of a world with a common language which everyone can speak. A world where you could go to China, Korea, Russia, Japan, or anywhere in Europe, and communicate effortlessly with the International Language. Is this realistic? Will it ever really occur? It seems like something of a pipe dream, but it would certainly be awesome.
FURTHER READING
Spaced Repetition Systems
Sentence Mining
Five Reasons to Study a Foreign Language
Autodidact: Be A Self-Teacher
Six Reasons to Learn a Language Together
Progressive Training
Will the Languages Of The World ever Merge?