From the periodic table we know Tl has 3 outer electrons. As it has a 2- charge this means it now has 5 on the outer shell, allowing it to bond with 5 other atoms, in this case Br, hence TlBr52-
The shape for an ion with 5 atoms bonded to it is trigonal byprimdal.
You just learn which shapes molecules have given the numbers of bonding elections they possess, so 5 bonding pairs will give a trigonal bypyrimdal shape, 3 will be trigonal planar, 2 will be linear and so on (not accounting for lone pairs)