I don't
I keep wanting to learn a language and have free access to many avenues (rosetta stone, rocket language, transparent language online from the military) but I really can't decide on what to learn.
First question to ask yourself.... Why do you want to learn a language?
Second.... How much do you want to use it?
Third... How much time are you willing to put in to learn?
Fourth... Is this just a hobby or do you want this skill to have real utility?
Depending on your utility for the language, your interests and the difficulty you're willing to endure will determine which language is best for you.
I wanted to learn Spanish because I love soccer, so being able to understand the commentators during a match was enough incentive for me. However, Spanish wasn't the first language I wanted to learn.... French. At 9 years old, it was too difficult to learn it on my own plus I had no access to it outside of audio/video tapes & books. French isn't widely spoken in the US, but it's the official second language of Canada. At that time, I had no way to utilize what I was learning, so I gave it up.
Spanish was easier and more accessible here.
If you want to get paid with your language skills, Arabic or any Asian language will make you money, but those languages are the most difficult to learn. Not saying it's impossible, but it's best to start learning these languages young.
Germanic languages are a little easier(not much though) because English, at its core, is a Germanic language. German and Dutch, to name a few, fall in this language family.
Next, is the popular Romance language family.... Portuguese, French, Spanish, Italian and Romanian. Widely accessible.
You can even learn African language if your heart desires.
There are many language families and the possibilities are endless.