Th Autodromo di Monza is situated in Monza Parc, approximately 3 miles from Monza station and about 10 miles from the Central Station in Milan. There are quite a number of hotels in Milan, varying from the very cheap to the extremely expensive and so is a popular place to stay for the GP weekend.
To get to the circuit from Milan by public transport is both cheap and easy. Trains run from both Central and Garabaldi Stations, there are more from Garibaldi. The journey time takes 17 to 18 minutes and the ticket price in 2012 was 2 Euros 10 Cents single journey. On arrival at Monza Station there are two choices, partially dependant on which day it is. On Friday, Saturday and Sunday the free Black Line buses run from a car park/bus terminus accessed from platform 5. This takes you into Monza Parc, this is followed by about a mile and a half walk to the stadium. On Thursday to Sunday there is a bus number 221, cost 1 Euro 55 cents, which takes you to Biassono, the stop is the one after the signs for the Ferrari car park. There is then about quarter of a mile walk to the F1 Village inside the circuit. On Sundays there are free trains every half an hour from Central Station to a station next to the Lesmo One curve, after the race there are trains back. Access to the return trains is a mixture of the order of arrival at the station and a bit of pushing and shoving. It seems to be more civilised than the usual scramble that takes place in Italy.
There is a pit lane walk on Thursday, it takes place from 16:00 to 19:00 hours, yes, three hours. It is by far the best one that I have been to. It is accessed from the F1 Village, it's just a matter of showing that you have a three day ticket multiple times.
There is camping available, one is adjoining the part of the circuit between the 221 bus stop and the village, the other is next to the Lesmo entrance. There may be others as well. There also seems to be unofficial overnight camping on Saturdays inside the circuit, some people just kip down in a sleeping bag or on the grass. The temperature is normally in the upper 20s (C) during the night so not much bedding is needed.