How to delete all text from beginning of buffer to mark

[Angus Comber]

I am in a large file of text and my cursor is at the start of the text I want.  
I want to delete all the text before the cursor.  I know I can set the mark 
using C-sp but how do I then select to the beginning of the file?

What is the best way to basically delete all text before the cursor in the 
buffer?