Apps for Android or Ios

DuoLingo has recently added Chinese to its large range of languages. It is the best app I have tried so far, having been right through Hello Chinese and a good way through ChineseSkill, among others.  There is an initial assessment for those who are not beginners.  This will start you at the right point without having to go through the basic stuff yet again!  You can do the whole course for free, though there is an option to pay about £50 a year to go ad-free.  This also enables you to download lessons for use off-line.
There is also a web version at www.duolingo.com/course/zh-CN/en/Learn-Chinese-Online

Hello Chinese is a great app to get going with.  There is some useful stuff near the end, though it doesn't get quite so challenging as DuoLingo.  It features useful touch-screen exercises for learning to write the characters.  There is no Web version, but there is a website at www.hellochinese.cc/