Lingodeeer, duolingo and hellochinese and chineseskill

Have a look at : http://www.omniglot.com/books/language/chinese.htm...

I'd try Duolingo.

I tried Rosetta Stone and I didn't like it.

The biggest problem is that it doesn't train you to speak, and since Mandarin is also a tonal language, it's very important practice speaking.