I need some advice, I got a data file to analyze for someone, and it appeared to be a time series data, I ain't very experienced with time series models.

My dependent variable is continuous (we'll call it Y). It was measured once a month, 12 months a year, for a few years. My first case is in the beginning of 1998, and the last one is the end of 2009. For every case I know at which month and year it was measured.

I have independent variables. They are all continuous, I have 7 of them.

If it wasn't a time series, a linear regression model would have been a good choice to start with. Since it's a time series, there is the problem of correlation between the cases. Maybe also a problem of seasonality. What should I check, which time series model should be used ? If there is a need to define ACF, how do I know which one is suitable ?

One more thing, if I create a categorical variable, of the "season" (spring, summer,...), can I use it to control the seasonality in the data ?