First welcome to the site!
People here will gladly help you spend your money so be careful!!....
Couple things to consider on a lift and bigger tires. You state you have a Z66 AV and I think that comes with self-leveling rear shocks and that may cause issues with air lines and other parts fitting properly. It also affects getting you up into an airstream and could have some effect on gas mileage due to air resistance as well as more rolling resistance of wider/bigger tires. Also if the tires are a larger diameter you will need to reprogram your speedometer or you mileage/speed will be off and could lead to a ticket.
As far as performance mods there are some things you can do but you have to realize the AV is a 6,000 lb. truck with the aerodynamics of a brick so it makes for a poor starting point for turning it into a high performance vehicle. Do you have the stock exhaust on it? Probably the best bang for your buck performance wise would be to get a good CAT Back exhaust system like Magnaflow or Corsa Sport but some performance can be gained from systems like Flowmaster as well they just usually don't come with a lifetime warranty. Do yourself a favor and don't get sucked into the advertising claims on things like a cold air intake, they do make it sound more aggressive but generally won't give you the gas mileage or performance they claim. Some members have gotten improved performance from a programmer.
A lifted AV may cause parking problems with parking garages too.
Being just 18 and starting college you may just want to consider saving your money since you don't know what expenses you may have in college or in your dating life.
Good luck and I hope you have many happy and safe miles in your AV!