If you are really serious about learning Chinese, the best way to learn is to go to China and get immersed in the language.
There are many websites for learning Chinese.
You can try: Duolingo.com
With learning a language one of the most effective ways to learn is to practice with a native speaker. Listening, speaking, and continual feedback are needed to help develop ability and confidence.
You can look at Italki – Learn a language online for one on one teachers.
Or BLCU Beijing Language and Culture University if you’d like online Chinese classes on skype from a Chinese University.
If you want to learn Chinese to a very high level
Living and studying Chinese in popular places such as Beijing and Shanghai are great places to study, but if you find that your goal is to learn Chinese you may find that you spend a great deal of your time speaking English to other foreigners if you are not careful and the temptation to speak English with Chinese friends who want to practice English can be a constant battle that needs discipline to overcome.
You can counter this by living with Chinese roommates, and forcing yourself to speak Chinese at every opportunity, even if they speak to you in English, or by making special effort to spend more time with Chinese friends. Ultimately your results will depend on how much time you put into it and your discpline
You might want to consider an intensive program such as the IUP Berkeley Tsinghua Program or another program that doesn’t allow you to speak in English at any time! The cost might seem more expensive, but it can be worth spending a bit more to get faster results. You can see very fast results from this kind of immersion. Living with a Chinese family, and in a remote area can also help to get better results, you would just want to be careful you don’t pick up a remote or strange accent!
You can also take extra classes in your spare time if you want to accelerator your learning, listen to podcasts, and watch Chinese tv.
Here is an interesting video with a student who shares some ideas about how to learn Chinese