Tinks, you may not be able to do this via a Twitter mobile app. When you get to a computer, log in to your profile through www.twitter.com, go to your profile, select "Account" and then click the "Protect my Tweets" checkbox. Then, unfollow and/or block all of your followers whom you don't want to follow you.
Thereafter, people whom you haven't permitted access to your tweets etc will see your profile as "locked" i.e. with a padlock next to it. They may send you a follow request which you may accept or reject as you see fit...