Photography can be a huge money pit. New cameras, lenses and tripods come out every year and we're all tempted to go out and by the latest and greatest.
Besides the camera and a solid lens, you only need a few good quality items to step up your photography game.
External flash: If you shoot with a flash you learn your on camera flash doesn't cut it. You need one that can be pointed in a direction other than at the subject. The light becomes more diffused and you have a lot greater control over the look of your shots. Getting an external flash allows you do a whole range of things you couldn't do without it. Including placing the flash behind the subject as a back light.
Tripod: A tripod is the only way to get certain shots. Long exposure shots don't happen if your camera isn't locked down. Even if you're taking portraits, it's nice to be able to direct your subject and come back to your camera ready to go. You don't need to spend hundreds on a good tripod either. I had one last 4 years and I paid $40. Money well spend.
Photo editing software: Taking the picture is only half the process. The other half takes place in your photo editing software. Fine tuning colors, adjusting brightness and switching to black and white are all things that should be handled by your computer.
Filters: I have 2. A UV filter which acts as an invisible lens cap, protecting the front element of my lens. It's 20 bucks and good insurance for your expensive lens. The other is an ND filter, which is like a pair of sunglasses for my lens and allows me to take long exposure photos during the day. If you think you'll shoot long exposure get an ND, if not then don't bother.
Lens Cleaning Kit: You'll need a way to remove dust from your lens and inside of your camera. I use a Rocket Blaster, it blows a jet of air and it's never touching your camera so you won't scratch anything.
That's it. Each item allows you to do something you couldn't do without it. In the end it's about taking pictures not fiddling with gear. So don't stress too much about getting the newest or best stuff.