Set default open with and how to handle file?

How can I set a file extension to open with my application?

Also how would I handle the file that it opens with?

1st
Open Explorer and rightclick on a file with the extension you want to associate with your application. In the contextmenu, click on 'Open with...' and take it from there

2nd
Check out the My.Application.CommandLineArgs property on MSDN

Well I mean by code, not doing it manually. Like those 'associate all .bla with my app'

Ok, well in that case, take a look at this link