You should get a letter telling you the IBI amount in the post, this is normally in August for payment in September in our area (Costa Blanca) but it varies as to were you are in Spain, your lawyer should set up a direct debit for you when you buy the house but this does not always happen
The owner of the property on the 1st of January will be the name on the bill for that year, it will go to your name the following year
If you go to your local town hall they can help, if you are in the Costa Blanca then you need to see the SUMA as they handle all the taxes there, you can phone them in english on Monday, Tuesday and Wednesday in the morning and they are very helpfull
If you have missed a payment there is a fine added but its not the end of the world