The code for Global.asax is stored in a code behide file Global.asax.vb - which can be found in the APP_CODE folder below the root directory.

However, I would suggest that you do a rethink on how you want your hit counter to work. 
You should avoid making any changes to core elements in DNN as any new core updates would potentially break your code.

I would look at making a SKIN control that stores its info in SQL.

What in particular were you thinking of coding into the session_start() method.

Westa