Here you go But I would not suggest to auto-follow your followers because you must see if the follower is really a good person or just a spammer? This article may help you understanding When & Who To Follow Back On Twitter Good luck.
1st link does not work as its redirect to other homepage. 2nd link works gr8 as this clear and detail is given.
You could use tools like Hummingbird to auto follow your followers, but I guess you would really want to check first who is following you and whether they are worth following back or not.
You join socialoomph.com. The service that you want is free. It also has auto-unfollow for people who unfollow you, and automatically sends a DM when people follow you.