I remember when I graduated University and found that retro t-shirts and ripped up jeans were no longer acceptable in my transition into adulthood. My advice is to invest in key pieces of your wardrobe that are timeless and will take you a long way. For instance, try to find a great fitting pair of black pants, and great collar shirt, a cocktail dress, some chic sweaters, and a great pair of boots. It's okay to also keep or find some fun prints as well for those girls night out and evening events.
For those key pieces, don't be afraid to pay a little extra as they will last you a while. When it comes to the accessories and fun tops you can afford to be a little more frugal here.