Public transport: here is a link to Milan public transport site. If your plane lands in Linate
Airport, you can take bus number 73. Bus tickets are available at
Linate newsagents.
Here you can find a map of Milan underground.
If you land in Malpensa Airport, you can choose to get to Milan with
Malpensa Shuttle, a bus that takes you in Stazione Centrale, or with
Malpensa Express, a train that stops in Stazione Cadorna. Both stations are situated on Line 2 (Green Line) of the underground.
Check
this if you need to get from Orio al Serio Airport to Milan, or
this page if you're travelling by train.