You'd have been better in the 'Business & Finance' section for this.
1. Your 1st tax return will be for the year ending 5th APril 2009 and must be filed before 31st January 2010.
2 . You only ever file a tax return annually. However, if you are making enough money doing it you might be required to make tax payments on account twice per annum. (January and July).
3 - You would inform the revenue on the tax return (or you could write to them specifically). It won't necessarily mean they stop sending you a tax return to complete though and, if they send you one you MUST complete it, you don't only do so if you have non-employment income.
4 - I'd need to check but I think you can either take an accepted percentage of the rent as repairs or actually total up genuine expenses to arrive at a figure if it's more advantageous. It's a while since I've done such a tax return, I used to do them for my sister when she rented her house out but she sold it about five years ago now, You aren't "taxed" on your mortgage at all (How could you be taxed on a cost?) but you used to be able to claim the interest element of your mortgage payments as an allowable expense against the rental income. Not sure you still can though.