If you want to share a post on Instagram, you have two options available. One, to take a photo with Instagram and the other uploading one from your phone's photo gallery. To take an Instagram photo, simply tap the "Plus' icon located at the bottom-center of the screen, or to select one from your phone. From the camera, you can switch the photo grid on and off, switch between the front-facing and rear camera, or choose whether or not to use the flash. You can tap the big round button in the center to take a photo or hit the video icon to record a video.
Once you've chosen a photo, you can add effects from the editing tools at the top of the screen or select a filter from the choices at the bottom of the screen. Then you can write a caption for the photo, tag people in the photo, or share it to Facebook, Twitter, Tumblr, or Flickr. When you write the caption to your photo, you can use #hashtags and @mentions. Mentions let you bring posts in front of other Instagram users who will get a notification when you mention him or her in a post and hashtags will help other Instagram users to find your posts, or help you to tag or categorize your posts for yourself.
To tag people in a photo, tap "Tag People" and tap on someone in the photo. Start entering their name or username, and select the correct users from the drop-down menu that appears. If the person that you want to tag doesn't appear in that menu, tap "Search for a person" to find him or her. If your photos are public, then anyone can see the photo, and the person tagged in the photo will get a notification. If your photos are private, then only people who are following you will be able to see the photo. The person whom you tag in the photo will get a notification if they're following you. (You can see and manage the photos that other people tag you in from your profile.)
When you post your photo, it will appear both on your profile and in your feed. If you've set your profile to private, only the people whom you've approved to follow you will be able to see it.