Reddit mentions of Coca-Cola "Waitress" Framed Logo Mirror

Well, for some basics....

Pick a color scheme. Easiest way to do this is to pick a main color and an accent color. Whatever your primary color is, you'll want your curtains/upholstery/bedding/rugs to conform to it, and the accent color can come into play in wall hangings, knick-knacks, etc. Each room can (and probably should) have its own scheme, but some re-usage of colors is fine.

Frame any posters or pictures you want on the walls. That goes a loooong way toward making the place feel like an adult lives there. You can get cheap frames that look nice at Michael's, or even Walmart.

Ikea is the default "cheap but nice" furniture place, but you can still rack up a huge bill there. Keep a close eye on craigslist and be patient. If you wait for the right deals you can save lots of money, even if it takes you some months to acquire everything you want.

Curtains and throw pillows. Get them.

You can try to "theme" certain spaces. For example, my kitchen is full of coca cola stuff, and my living room has several nerdy vintage travel posters. Even if you don't dedicate a whole room to a theme, you can do small areas.

A full set of floor rugs for your bathroom is a good idea. Get a matching shower curtain too.

Finally, and perhaps most importantly, keep the place clean. Vacuum, pick up after yourself, and don't let the kitchen become gross. All your work decorating is for naught if there's garbage everywhere :p

Good luck!