1. Create a new file (cmd-N)
2. Save as with an extension of .css (cmd-shift-S), name foo.css (or whatever).
Or, without saving:
1. create a new file
2. use the little drop menu at the bottom of the frame that says
'(none)' and select CSS
Or, from terminal:
1. `touch foo.css`
2. `bbedit foo.css`
This of course applies to pretty much any file type.
There probably are even more ways, but these are the three I find the