There are numerous books available to help you learn how to write in Hindi. For example, the Teach Yourself Hindi series by Rupert Snell is excellent, and for a lucid primer on Hindi grammar, refer to the series of texts by Usha R. Jain. Those books, and some others, also come with an accompanying CD set to provide oral guidance on the sounds of Hindi letters.
Bollywood movies, especially those on DVD, offer another effective method for learning how to write in Hindi. First, hearing the Hindi words in general will familiarize you with the sound of Hindi words and make it easier to spell them out as you learn the Hindi alphabet. Second, Hindi DVDs typically come with subtitles in Hindi, allowing you to pause the movie as it runs and match up the words the actors say with the Hindi words on the screen. Bollywood produces a greater number of films than Hollywood, ensuring that you will have a constant and ample supply of prospective films to watch. A standard retailer for purchasing cheap, quality Bollywood movies is Moserbaer.
There are ample online sources to provide assistance in learning Hindi. For example, BBC Hindi provides daily news in article and video form entirely in Hindi. The Five College Center for the Study of World Languages also has a comprehensive set of dictations, written assignments, videos and other materials online available for learning how to write in Hindi.