To enable debugging, open the web.config file with notepad, that is located in the volume containing your Inetpub root. Eg:

c:\Inetpub\wwwroot\wss\VirtualDirectories\80\web.config

Search for the Callstack attribute. Change it from False to True.

Then locate the CustomErrors element. Disable it.

Check any web parts or controls are registered in the Safe Controls list.

Finally, add a new element to the web.config file:

<trace enabled="true" pageOutput="true" />

Now you will receive the full stack traces as output on errors instead of the cryptic non-descriptive error message you get by default.

To debug a web part, under the Debug menu select Attach to Process. Check the Ensure Show System Processes and Show Processes in all Sessions boxes. Locate the w3wp.exe process that is running when you have a browser open with the site in it. Select this process to attach the part too. Back in the browser, access your web part as normal, and when it encounters a problem or a breakpoint, you will be sent back into the Visual Studio debugger.