I can't take a picture. It reads on the top right

The F means that the lens is not reading properly. The reason for this depends on the lens, but it's usually because the contacts between the lens and the camera are dirty. Remove the lens and clean them with a cotton swab dipped in alcohol.

If you're using an older lens, you may also have put the lens in manual aperature mode. You should select the aperature marked with the red if you have an old lens. (New lenses won't allow you to set aperature with the lens...so this isn't a problem.)