Without knowing more details about your vehicle, your overall goals and use, and your budget.. A quick and safe shock recommendation would be Bilstein 5100's. They're usually a pretty nice improvement over stock, especially if the stock stuff is worn, and are very reasonably priced. They're pretty nice for a mix between daily driving and mild offroad.
That said, even though they are likely a safe bet, I would do some research (searching this and other forums, and general google searching) for more info on them with your specific application. I've not owned a Silverado (only a Suburban) so
What have you done so far, if anything? Based on the pic, it looks like you're 'leveled' at least. What year is it, 2004? (wait, based on your SN I'm guessing '03, duh..) :p
Good AT tires and a set of better than stock shocks should be a pretty noticeable improvement and a good foundation until you learn more. I wouldn't recommend rushing into buying a bunch of cheap suspension parts as you will probably end up just replacing it with better stuff later. Also, depending on condition, you'd probably notice a lot of benefit from going through and replacing and servicing some of your worn stuff, depending on mileage. Bushings, lube points, etc.
