mortgage- rental income

i was wondering, say if i had a property on my name, a house and i rented it out would that be included as income say even if its used to pay off a mortgage ? If so, when could i request for that to be taken in to consideration. Say if i by a property next month and rent it out the following, and would that be enough proof or nt ? Would the proof need to be off 6 months or 1 year ? Any answers or suggestions much appreciated