Locate and open the target SharePoint application's Web.config file. By default, it is located in C:\Inetpub\wwwroot\wss\VirtualDirectories\80.
Find the following line of code and change the debug attribute to true.
XML
<compilation batch="false" debug="false">

Save the changes to the Web.config file.
In Visual Studio, right-click the SharePoint project, and then click Properties.
Click the Debug tab, and then type the target SharePoint URL in the Start browser with URL box.
Place a breakpoint in the code, and then press F5.