I have a problem with my current research project and I am not very experienced with statistics so it would be great to find some help here!

I have performance data of companies which can be categorized into two different groups (state-owned, not state-owned). Now I would like to analyze the effect of a certain event by using data before the event happened and afterwards (t-1 and t+1). This would be my treatment group.
To eliminate factors which are not associated with the event I have the same data for a control group (where the event didn't happen).

Similar studies used a difference-in-difference regression with the state-ownership and treatment group as dummy variables, but I just don't understand how to, especially not in SPSS.

I hope someone understands my problem and might be able to help me.