If you've set your Instagram profile to private, new followers will need approval before they can see what you've posted. Your phone should notify you of new requests. You can always check your Instagram news feed to view recent follower activity.
Checking Instagram News Feed
Open the Instagram app on your phone and click the news feed icon; as of January 2014 this icon looks like a message cloud with a heart in the center, located to the right of the camera icon. In the You section of the Instagram news feed you'll see recent comments, likes and follower requests.
Approving a Request
To approve a follower request, tap the username of the person who requested to follow you. On his profile page you'll see the notice "This users wants to follow you." Tap the check mark to approve the follower or the "X" to deny.
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