Buy a good textbook with a CD, such as "Barron's Mandarin Chinese: The Easy Way." Also take a language class. Once you have a good grasp of the basics, watch Chinese TV and movies and/or listen to the radio online as much as you can. You don't need to understand everything. Doing such things will help you get used to Chinese tones.
Take the Chinese Proficiency Test. (This is China's official and standardized test to assess the Chinese language proficiency of non-native speakers. It is the Chinese equivalent of TOEFL or IELTS.) Get a degree in Chinese; join a study abroad program in China read the biography of Mao Zedong in Chinese; join the speech contest of your local Chinese organization or Chinese school; memorize Chinese poems or sing Chinese songs. If you have a specific purpose for learning Chinese, you can focus on the vocabulary you need to know.
Immerse yourself totally in the language, the culture, and its people. If you are a grown-up, this is the fastest way to learn the language. Improve your speaking, writing, reading, and listening skills all at the same time.
As you learn to write Chinese characters, learn them one by one and write each one over and over. Use flash cards. Trying to write and read Chinese is a lifelong process, as there are thousands of individual characters.
Unfortunately, there is no easy way to learn a language unless you grew up with it. But that doesn't mean you cannot find an effective and enjoyable way to learn Chinese. Make friends or pen friends. Join Chinese groups or organizations. Learn the Chinese tea ceremony. Visit your local Chinatown. Find your favorite Chinese restaurant. Be a Chinese New Year or moon festival volunteer. Read children's or young adults books. Immerse yourself in Chinese history. Once you have mastered the language, make sure you keep practicing it regularly. If you don't use it, you will lose it.