How to get twitter data using firehose api?


Hi all,
In my project I need to extract data from social networks (Twitter) for analysis, please how can I proceed this extraction, I have to use what tools and what API for extact and I need to store all the tweets in json files, how can I do.? is it firehose api publicly available for extracting the twitter data or we need to take authentication from twitter for using this api?

Thanks and Regards
Nitin Dubey