how do you switch to the boomerang in 1-1

Once you collect ammo (the drops that have little rocket-like things on them that enemies drop) you push start to cycle between your three sub-weapons: batarang, pistol and then dirk.

You can tell when Batman has ammo in reserve as in the top left corner, BATMAN turns from purple to kind of an orange-ish colour. Likewise, once Batman runs out of ammo (or has 0 ammo) it reverts to purple.