I want to buy clothes and shoes for myself with reasonable prices in Milan in early December. where can I buy them?
What do you mean reasonable? 100 € for a pair of shoes or 40 € ?
The classic "reasonable" (or cheap) department stores in Milan (and the whole of Italy) are OVS and UPIM. Then there is COIN in which you can find both expensive and reasonable stuff. For streets, you can go to via Torino and continue down to Corso di Porta Ticinese and maybe you will find something. Then the classic "cheap" street of Milan is Corso Buenos Aires, from Porta Venezia to Piazzale Loreto.
Tassos has given you some excellent suggestions. For your information, large branches fo OVS [ https://goo.gl/maps/oDLae7THEXU2 ] and UPIM [ https://goo.gl/maps/5o3HmB7GBm12 ] can be found on Corso Buenos Aires that Tassos mentions.
For more shopping streets in Milan, you can have a look at a forum thread from earlier this year from someone who asked "planned a day trip Milan... hoping to shop and find some great bargains" [ https://www.tripadvisor.com/ShowTopic-g18784… ].