time series modeling with one- lag

Hi all

I have a time series of a random variable ' a ' which varies both in month and year. Historical data of this variable for 12 months and 25 years i.e. matrix 25 by 12, is available. I want to write a program using such historical data and Auto Regressive (AR (1)) model to generate a series of data for long period. I've found thomas-fiering test for this problem but I don't know how it works. Could you please guide me.

Thanks in advance,