Things to know about Milan Transport System:
Milan has the best transport network in Italy. ATM is the company managing the network which has 5 metro lines (the M4 line, the blue line, will be completed this year; nowadays only part of it is open), many trams and buses that will bring you to any corner of the city.
Price: standard ticket is €2.20, and you can use the same ticket for the duration of 90 minutes and take anything, metro, trams and buses.
Where to buy tickets: In the underground they have machines before the entrance selling paper tickets, or you can go to "Tabacchi" or "Tabacchini" and they will likely have paper tickets too.
For me though it's not necessary to buy paper tickets, as you can also pay with contactless card, Apple Pay or Google Pay. Whenever I don't have a monthly travel card and need to pay one single journey, this is how I do it.
How to pay with contactless cards: In metro you need to validate the ticket when you enter but also when you exit. Otherwise, they will charge you more.
In busses and trams, the machine for contactless payments is usually located on the front.
Do you need a daily card: There is the option to buy a daily card and use all the means of transport for 24 hours without limits. You'll likely not need to take them many times, most places are within walking distance, but in case you do, just know it's an option. The 1 day ticket costs €7.60.
3 days tickets costs €15.50