Twitter has follow limits. That means per se that you can't follow more than a 1000 users per day. However, and more importantly, you can only follow 20% more than you have followers. That means if you have 3,200 followers, the maximum number of people you can follow is at about 3,900 people before the limit kicks in. (This particular limit does not apply if you are following less than 2,000 accounts.)
The other important limit is for new accounts. When you hit 2000 people that you follow, that's the end of the road for you, until all 2000 users you follow follow you back. Which will never happen, unless you do something about it.
So you want to find those people you have initially followed and that are not following you back after a reasonable amount of time, and then unfollow those.
So let's say you have found and unfollowed 200 people who are not following you back. You are now back to 1,800 you follow. That means that now you can follow again 200 new users. Give them a week or so to follow you back. After a week you go through the same process again.
How do you find those people who are not following you back? I personally use ManageFlitter, by far the best app out there to manage your Twitter account. They are a bunch of nice people in Australia, with a good customer service. The app doesn't cost anything. If later on you want to upgrade for special features like analytics, there is a button for that too.