What is Sinatra?
If you already have a sinatra app, use it or go ahead and create one and add below code which basically has sinatra routes.
#web.rb get '/' do 'Hey there!' end
You need a Procfile file alongside your config.ru to tell Heroku how to run your app.
web: bundle exec ruby web.rb -p $PORT
What this says is that run a web process on the port Heroku gives you using the bundle environment.
$:.unshift File.expand_path("../", __FILE__) require 'rubygems' require 'sinatra' require './web run Sinatra::Application
Array#unshift will prepend the new path to $:
File#expand_path is considered to be Ruby idiom for getting the absolute path to a file when you know the path relative to the current file.
Keep Coding !!!