Actually Nisan is considered the first month in the counting of the months of the year. Tisrei, when Rosh Hashana is, is the seventh month of the year. Throughout Tanach when a month is referred to as the fifth month, for example, it means the fifth month starting from Nisan.
We begin the counting of a new year from Rosh Hashana and not from the first month since that is when Mankind was created, the world was created - and everyone is judged. But the Jewish nation was formed in Nisan, and that is the first month in our calendar regarding the months of the year.
I hope this is clear and helpful and wish you a good week - and month - and year.