As a general rule of thumb, mastery over around 2,000 of the most common words will allow you to participate in most casual conversations.
Assuming knowledge of the Hindi script, Richard Delacy's "Hindi Flashcards" (published by Tuttle) presents mostly common, conversationally useful words and phrases. In terms of dictionaries, the Teach Yourself "Essential Hindi Dictionary" is the only decent one I've found that focuses on the most commonly used words.
Both of these, however, don't cover a lot of the colloquial language currently in use and fail to cover how particular English words and phrases are used in casual conversation.
