If you are only earning while you are working, ie cleaning this hostel, then you have a job [albeit only a small or part time job]
If you are earning in your absence, ie other people doing the cleaning, then you are on the road to having a business, again, only a very small one at this stage.
To take on employees, you need to register with the Inland Revenue and they will then send you out an information pack and also your employers reference number etc.
They will also send you information run by the Business Services division of the IR. These are basically 'free' consultants and they run free courses on all aspects of tax and employment.
If you look at the IR website you should be able to find all of the information you need.
Mick