If you want to have the easiest (not guaranteed) learning path I suggest you to take a language that is similar to yours or that belongs to one of these groups:
Italian-Spanish-Portuguese-French-Romanian = Romance
Although Romanian has an asymmetrical intelligibility with the other countries, making it easier for a Romanian to learn a foreign language than the opposite, it also name cases, which have disappeared in all other romance languages.
If you start from Italian-Spanish-Portuguese you can usually learn one of these fairly quickly.
French is the faggot of the situation by being the "oh so unique" romance language, but if you prefer go for it.
English-Dutch-German-Danish-Swedish-Norwegian-Icelandic-Afrikaans = Germanic Languages
English being the French gobbling whore it is has less than one third of its vocabulary actually being germanic, so you might want to consider french.
Dutch is similar because it borrows much from english and french and Afrikaans is a dialect of Dutch.
The Nordic languages are pretty similar to each other so start from one of those if you are from there.
Polish-Slovak-Czech-Russian-Ukrainian-Belarusian-Bulgarian-Serb-Croatian-Bosnian-Slovene = Slavic Languages
These further divide into three language groups:
West Slavic Polish-Slovak-Czech (Czech and Slovak being really similar). All use latin derived alphabets.
East Slavic Belarusian-Russian-Ukrainian. All use Cyrillic.
South Slavic Bulgarian-Serbian-Croatian-Bosnian-Slovene (the last four being particularly similar). Bulgarian uses Cyrillic, Serbian uses a mutt of the two, the rest use a latin derived alphabet.
Hungarian-Finnish-Estonian = Uralic Languages
This group isn't even an indo-european one, so try it at your own risk or if you're from one of those countries. Prepare for long-as-fuck composed words.
Lithuanian-Latvian = Baltic Languages.
Dunno much about these, but they are cousins to the slavic languages, although I don't know how much that can help.
Maltese-Arabic-Ebraic-Aramaic-Amharic = Semitic languages (if you wanna be a prophet, a bomb, a banker or a nigger)
Maltese is basically in the same situation as English as most of its language is composed by Italian words.
Jewish and Arabic aren't really that intelligible with each other but there are some similarities.
Each Arab country has their formal Arabic dialect, so one is different to the other.
Turkish-Azebarjani-Uzbek-Kazakh = Turkic Languages (Remove Kebab)
Same as Baltic, intelligible but can't tell you much more.
Mandarin-Simplified Chinese-Cantonese-All the other chinese languages-Tibetan-Burmish = Sino-Tibetan Languages
The Chinese Languages actually forming a branch of their own (the sintic one), also the chinese are going full "fuck the Tibetans".
THE ROGUES:
These have no other language in their group besides themselves.
Greek - This one is from a vocabular perspective the easiest to learn thanks to the fact that there's a bit of it in every single language. Thanks Classicism.
Basque - It's not indo-european either, but is even older. oldfag language
Albanian
Georgian
Armenian
Japanese - Fucking isolationists (they did steal the chinese writing system, though)
Korean - North and South Korean are slowly diverging though, thanks to gommunism
Mongolian - [throat singing]
THE DEAD
Irish-Scottish-Cornish = Gaelic languages.
You really wanna learn these only for historic reasons, as almost no one uses them.
But since you're a filthy american mutt I guess I've just satisfied my autistic instincts.