I’m sorry, the general public cannot get tickets to the fashion shows. The shows are reserved and by invitation only. The only way you can get tickets is to get invited as a friend, a member of the press or as a buyer if you work for a shop, boutique or department store that could be a potential client for the brand.
However, there are many parallel fashion events during fashion week which are going on in Milan which are open to the public. See the website http://www.cameramoda.it/en/ for other fashion happenings.
You may want to take advantage of the expertise of a guide who can bring you on a tour and show what’s going on and the trendiest places to hang out, from €65- Fashion and Cocktail Tour of Milan