The best way to learn Chinese quickly is to go to China on an intensive Chinese language course. Surrounding yourself in Mandarin all day every day will improve your skills faster than any other method. As such, staying with a host family, making Chinese friends, and forcing yourself to speak only Chinese will all speed up your learning even further.
Chinese is a difficult language, especially for beginners. The most intimidating things at first are the tones and the writing system. But once you start learning, you’ll discover elements of Chinese that are simpler than other languages. Numbers are extremely logical, verbs aren’t conjugated, and the complexities of gendered nouns and adjectives don’t exist. Getting beyond the initial complexities quickly is a good idea, so you can begin to enjoy learning and speaking in Chinese.
If you’re unable to go to China for financial or logistical reasons, the next best way to learn Chinese quickly is to take an intensive course near where you live and supplement that with practice listening to Chinese (movies, TV, radio) and speaking Chinese (find a conversation partner). You won’t learn as fast as you would in China, but you can certainly make good progress if you have class a few hours every day, review your lessons afterward, and find a way to practice using your new skills.
In any case, learning Chinese quickly will require time and motivation, so it’s best to set out with reasonable expectations. While you can learn basic Chinese in a month or two, to become fluent enough to work in a Chinese company, you’ll need at least a year of intensive study.