。
I forgot to lock the storage shed.
Literal
Storage-shed [target-に] lock [object-を] put [nominalizer-の] [object-を] forgot.
鍵をかける means 'to lock' (literally 'to apply a lock'). ~のを忘れる means 'to forget to do X.' ちゃった is a casual contraction of てしまった, adding a sense of regret — 'I went and forgot.'