There are countless books and blog posts written on the topic. The entire "Lean Startup" book by Eric Ries is an answer to your question.
Have you read them?
If not, go read them.
If yes, http://www.catb.org/esr/faqs/smart-questions.html#before
Have you read them?
If not, go read them.
If yes, http://www.catb.org/esr/faqs/smart-questions.html#before