As Keith says getting to Monza from Milan by train is easy. The trains run from Milan Centrale Station and were (not sure if still the case) free on Qualifying and Race day. It is very busy and can be a bit manic at times but adds to the experience. Once you arrive in Monza station you follow the signs with the circuit outline on them which take you to the free buses that run from outside the station. These take you straight into the park, however these drop off at a central point and then you have to walk for circa 20 minutes from there to the circuit entrance, then it is another 20 minutes roughly to get to the entrance gates (it is in park remember.) Some of trains also go to the station at the circuit, but only a few, so if you get lucky you may get one of those, othewise just to Monza itself.
I agree with Keith about Monza. It is probably one of the cheaper races to attend in Europe but the atmosphere is amazing. Food wise, yes would agree again. Decent quality and not to ott re price.