After all these years, and growing up with Windows 3.1, I have seen an entire evolution of computers and software. Touch screens and large resolutions were a pipe dream just 15 years ago. Now it's the norm. Going from a Packard Bell (yes, before HP) that couldn't run 3D Ultra Mini...
Shoot everything at least once. That's how you earn the big bucks.
Clearing waves gives you bonuses, so try and kill everyone within the time limit.
Shoot barrels when bad guys are near them for quick explosive kills.