You can set the server Sinatra uses with the :server
configuration setting:
set :server, 'webrick' # or thin, mongrel
In production, it is much better to use a more sophisticated server like Phusion Passenger or Unicorn, since they have better performance than Thin, Mongrel, or WEBrick. If you choose Passenger or Unicorn, you would not configure the server within your Sinatra application file itself, but instead typically configure it separately using a Rackup config.ru file.
"Ruby on Rails Server options" is aimed at Rails applications, but is still very relevant for Sinatra apps.