You say it 3 ways: + two thousand and fifteen [British English BUT less common now] + two thousand fifteen [American English BUT less common now] + twenty fifteen [General English AND the most common nowadays]

Without an.

If its a year- Twenty fifteen or two thousand fifteen. If its a number- Two thousand and fifteen.

I would agree with Jo Leesley. As a British person it depends on whether you are referring to the year or the number. If the number it is two thousand AND fifteen, if the year then twenty fifteen. However I have heard Americans leave the number out when referring to the number.

Hello Hafsa! I definitely agree with Adam - can say it all of those ways.