Always start with customer interviews. It sounds obvious, and even the people that do them can mess things up by pitching to early or leading customers to the answers they want to hear. THe best practice is to usually ask about past actions or behaviors rather than 'what will you do' or 'what do you want'. The term is called New Years Effect I believe, because obviously if you ask someone about getting in shape they will lie based on what they want to do, but may have never done before. Another great read is the book 'The mom test', to carry out realistic user interviews