Brushing should not be done on tooth extraction site till the healing occurs. If it is a normal extraction then after a 7-10 days but if it is surgical extraction then it may take long about 3 weeks. But make sure to make that area clean othewise food lodgement can cause the secondary infection and for that you can do warm saline rinses 2-3 times day or you can use some good antimicrobial mouthwash to keep area clean. For more info do's and don'ts after tooth extraction you  can read this article... http://www.identalhub.com/article_do's-and-don'ts-after-tooth-extraction-153.aspx