The quickest and easiest way to learn English is to take an English course abroad. Immersion environments make for faster and more natural language learning because in addition to class time, you get constant exposure to the English language. People speak English in lots of countries, so there are plenty of places you can go for immersion in an English-speaking environment.
The underlying assumption of your question is that people can learn English quickly and easily. A few simple tricks and you’re there. Right?
The reality is that learning a language takes time and effort. There’s nothing quick or easy about it. Sure, you can pick up basic English in a month on an immersion course, but to reach the level of English you’d need for an English-speaking workplace, that’s a serious project.
But so how can you learn English as quickly and easily as possible, given the reality of language learning? The first part is easy. You’ll learn more quickly if you’re studying with more intensity. It’s the same as someone learning to play tennis: if he takes lessons for 2 hours a week, he won’t improve as fast as if he takes lessons for 2 hours a day. So to learn English quickly, intensity is key. You can go on an English immersion course, as mentioned above, or you can study intensely at home or online. Anything that gives you more daily exposure to English and more practice speaking English will speed up your learning.
The second part is trickier because when people say easily they usually mean without much work, and learning English always requires work (unless you’re a baby). If you want to learn English quickly, you have to study hard. If you want to learn English easily, you’ll want to relax. So learning English both quickly AND easily is basically impossible.
Unless, of course, if when you say “easily” you mean “in a nice environment.” In that case, by all means, choose an English-speaking destination that you adore (somewhere with a beach perhaps?) and go there for 6 months on an intensive English immersion program. You’ll still have to work hard, but perhaps the sunshine and the soft sea breezes will make your English learning feel a little easier. It’s certainly worth a try.