We have a tenant who does not pay the rent of the flat: what can we do? How must we act?

We have a tenant who does not pay the rent of the flat: what can we do? How must we act?.Unfortunately, this situation has been raised once in the life of almost all those owners who rent an apartment, office or commercial premises. All the business have his risks and in the business of the rent, one of the risks more evident is the non-payment of a quota of the revenue reminded in the contract. There are other risks, such as damage that often cause to the property, but the unpaid rent is more evident because the rent is normally charged every month and if a month is left to collect, jump alarms.