What to expect at Katy Perry concert?

Yup me and my friends got tix to see Katy Perry!!!! Im so excited. But the seats are in the very back (and I mean in the BACK) of the place. Will it still be good? Like I guess we'd see her on the TVS and stuff....Do they sell tshirts and merch over there? And does anyone know if she does a fan greet b4 the show? The show starts at 7:30 so what time should we get there?