So, you just forked over the money for your first D-SLR. If you're relatively new to photography, there are a bunch of things you'll need to get the most out of your new purchase. Here's a list of essential things you'll need to make your new hobby more rewarding and productive.
Note: I'm posting Canon products because that's what I use. Choose what's right for your camera, Nikon, Pentax, Leica, or whatever.
1. Memory Cards. I'm sure your camera came with a memory card. Unless you bought it as part of some bundle purchase, I'm also guessing it's probably something like a 4GB card which means you'll probably run out of space on that card before you make one lap around your block. Depending on whether you're shooting so your images are saved as JPG or RAW, you're going to need more. Starting out, taking 'snapshots' to get the feel of your new friend, JPG is probably ok. It's a smaller file size so you'll get more photos on any card. RAW files are larger and require more space. Some camera's allow you to shoot both simultaneously (which I never do). RAW will afford you more options when editing your photos after and be able to accomplish more 'darkroom' techniques. If you're just snapping pics to transfer to your computer & then post online, JPG will work for now.
All that said, you'll need a bigger memory card. 32GB is probably good. You'll get over 1000 JPGs on that. Large capacity cards are fairly cheap these days, so if you can, get as much space as possible. I've never had issue with multiple 32s, but if 64GB is an option, go for it. If you buy multiple cards, you should get a case for them. Helps stop you from losing the damn things all the time.
2. Extra Batteries. Before you even run out of memory card space your battery is going to die. You always want extra fully-charged ones on hand. For a long day out shooting, It's probably good to have 2 or 3.
3. Photography Book. You spent the money for a D-SLR, you should probably make use of its features. Get a good book on the basics of photography (or you can Google stuff). I like the Tom Ang books. He's a seasoned photographer and pretty good instructor. HIs Master Class books are pretty good. They go through all of the basics in easy to understand 2-page sections, each with a small exercise to help you learn & a 'test' at the end of each chapter to put it all to good use. Soon, aperture, shutter speed, depth of field, and ISO will all be second nature knowledge.
4. Camera Bag. A bag is necessary if you're doing extensive shooting or traveling around. It's a subjective thing based on what you're style and habits are, how many lenses you might have, how much gear you're carting with you, etc. Get something that works for you. Does it need to hold everything? Just the camera and a battery/memory card? Shoulder sling? Backpack? Does it need to hold a tripod? Take your time and try out a few styles at your local camera store before choosing. I can't even count the number of bags in all shapes & sizes I've bought over the years.
5. Cleaning Supplies. It may seem obvious, but I have to say it anyway. You need to keep your optics clean. Both ends of the lens and the sensor. You don't really have to worry about the sensor too much at first because you're not likely to touch it. You're lenses? You should check and clean them before and during any photo excursion. A lens pen is your best choice. Brush away any dust first and then use the other end to 'polish' away any spots or smudges on both the outer and inner lens. Be very gentle. An air blower like Rocket Air is a good tool to help remove dust from the camera internals. With the lens detached, hold your camera with the lens opening facing down and blow out any dust without touching anything inside. Always keep your lens caps on when not shooting or when changing lenses. Dust sucks and can ruin your day when you open photos on your computer to view your master work.
6. Lens Filters. At the start you really only need a UV filter. Mostly these come in handy to protect your actual lens from dust and scratches. Better to replace a scratched $40 filter than a $1500 lens. Get one for every lens you have. As you progress with your skills you'll encounter other types, such as polarizing filters, star filters, and neutral density filters. All of which you'll learn about as you read your Tom Ang book.
7. TriPod. At some point you're going to want to take your time composing that skyline or sunset shot. The camera needs to be steady. For this you'll need a tripod. Again, this depends on your shooting style. Do you need a heavy one so it doesn't shake in the wind? A light one for backpacking up a mountain trail? There are also different types of heads that connect to your camera. There is a ball-head style, which lets you swivel the camera around a lot. There's 3-way pan and tilt which can give you more options. Entry level tripods usually come with their own heads attached. As you grow you'll possibly want to get one with more options. One great thing to look for is a level on the head, to make sure your photos are level with the ground.
8. Flash. Depending on the camera you bought, if it's a more entry-level D-SLR, you most likely have a built in flash. Some of those will also have a hot shoe. Higher end models won't have a built-in and only have a hot shoe. A hot shoe is where you connect a flash. (and a few other accessories). A basic speed light is a pretty good starting choice. Most brands make a low-end version that's fairly inexpensive. Again, as you go through your photography book lessons, you'll learn more about the capabilities and methods of using an external flash.
9. Diffusor. You spring for the fancy external flash. One thing you'll learn right away is that sometimes the damn thing is just too bright. You can fix this with a diffusor, There are tons of different types, large and small. You can get a simple plastic one, or a mini soft-box. These basically help your flash not blow out (over-expose) your photos too much. Some flashes even have a rudimentary one built in.
10. Photo Editing Software. You bought a D-SLR. You probably don't want those images sitting on your memory card or on your computer's hard drive. You'll need some kind of software to go through, categorize and edit your photos. Many cameras come with some software. You can try those out first, or go for something a little more robust. Photoshop and Lightroom are great choices and used by many photographers. You can also get a Creative Cloud account from Adobe that caters to photographers. There are a ton of other software choices out there. You'll have to choose what's best for you. I've been using Adobe's products for over 10 years.
11. Remote Trigger. You're all set up to get that amazing sunset shot or capture a starry night sky. The camera is ready and on a tripod to keep things steady. Then you have to press the shutter which makes the camera shake. Avoid that by getting a remote trigger. They come in wired and wireless version. Nicer triggers also have digital features that allow you to set up time lapses and other fancy things.
12. Lenses. Here's where the money pit happens. You probably got a lens with your camera. Something like a 24-105mm zoom. That's probably going to be fine for a while. Depending on what kind of photography you plan to do, certain lenses will help you out. Shooting mostly daylight? you can get away with a f/4.0 aperture lens. Low light?You'l want a f/2.8 or even a better, an f/1.4. Lower the F-stop number, the better the lens is in low-light situations.
Doing incognito street photography? You might want a smaller lens to make your gear lighter.
Shooting Landscapes or even interiors? You might want to go for a wide angle lens.
Perhaps you're looking to get great photos of wildlife or your kids playing sports? You want a good zoom lens that lets you get get shots while far away from your subject.
Maybe you like taking pictures of flowers or insects. For that you'll need a macro lens.
There are many makers of lenses that will fit your particular camera. Tamron and Sigma are two of the more inexpensive brands that make lenses for both Canon and Nikon.
Don't rush to buy just any lens. Make sure you get the right one for your preferred photography style.
There you have it. You should be ready to go now with your new camera. Take your time to practice and learn what you and your camera can do and find your own eye as a photographer. You may not like the results at first, but as with anything, it takes time to get good at it and "find your eye".
Have any more suggestions for what a new photographer should have in their bag? Add them in the comments.