Please note that the shuttle buses from Monza railway station to the circuit are not operating in 2021. Instead, you can take the Z221 bus from outside the station to Vedano (close to main entrance) or Biassono (top of the circuit, near Lesmo).
The best way to get to Monza on Grand Prix weekend is to take one of the regular trains from the large stations in central Milan to Monza railway station, then a shuttle bus to the circuit. The downside is that this involves a lot of walking; 20-30 minutes from the shuttle drop-off points to the circuit entrances, plus anywhere from 10-40 minutes to get to your seat inside the circuit. A taxi (or Uber) from central Milan should set you back around €40-60, but there’s a chance you will get held up in traffic on the small local roads next to the circuit. Driving is another option, though be aware that the local streets around Monza are closed to general traffic on race weekend, and you’ll be directed to one of the many car parks around the outside of Monza park.