Above answers are really good (especially the "interactive" seating plan - wish I'd had this when I was younger) but it largely depends on what you're going to see.
For classical concerts (Proms) I used to favour sitting in the choir (when available), behind the orchestra, because then you got a good view of the conductor's facial expression! Sit on the east side (left side facing the stage) if you want to see the keyboard in works featuring the piano. Also bear in mind that some seats have a restricted view, more of a problem if you want to see a show.