You're looking for memory cards for your camera, and you notice there are a TON of options out there. What should you buy? What do you need to look for in a memory card, and where should you purchase this card? Is there anything specific to be aware of when you're purchasing memory cards?
There are a few things to consider when purchasing memory cards for your camera: speed, capacity, manufacturer, and dealer.
First and foremost, it's important to understand that memory cards can be a little tough to buy, because card manufacturers use different numbers to advertise the speed of the card. For instance, you might see a Lexar memory card marked 133x and a Sandisk card marked 20 MB/sec. It seems impossible to tell which one is faster, unless you know what the “x” rating means. The “x” rating is equal to 150 kb/sec, so a 133x = 20 MB/sec while 200x = 30 MB/sec. However, it is important to not just look at the advertised speed of the card, because that is simply the maximum speed of the card, and this is usually not the speed you will be getting. You also need to see what the minimum is, and to do this you will need to look at the class rating on the card: a class 10 card is faster than a class 6 card.
In your camera manual in the technical specs it will tell you how fast of a card your camera can actually address. Make sure you see what your camera can do so you don’t spend money needlessly on speed you can’t use, but also make sure you have a fast enough minimum speed that you can do what you want to do. (Some camera manufacturers have worked specifically with the card makers to allow the camera to address a faster memory card, so it might be worthwhile to investigate this with your specific camera.)
Beware: a lot of photographers say “I don’t do video, so I don’t need as fast of a card.” This is wrong! Yes, video needs a fast memory card so you don't drop frames while you're shooting the video. 1 second of video can be 24-30 frames per second, which means 24-30 pictures are being taken at 1920 x 1080 resolution (which is tiny in comparison to what your camera can do). Alternatively, when you are shooting something like sports photography, you're taking shots just as fast as you can and you may be shooting as many as 6 frames per second. This 1 second of photography data stored in RAW format will need significantly more memory than the 1 second of video data. So don't pass up a fast memory card simply because you don't do video. Often, you will need a FASTER card for photography than you would need for video!
As for memory card capacity, there are lots of choices and you will need to determine how much storage space you are hoping to have. Shooting in JPEG will allow you to store 2-3 times more shots than shooting in RAW, but remember that when you shoot in JPEG you lose a lot of ability to edit your photos. The more photos you want to take at a time, the more memory you're going to need. And if you are shooting in RAW, expect to need 2-3 times the space you would need for your JPEG photos.
With regards to memory card manufacturers, stick with mainstream manufacturers (Lexar or SanDisk). It’s worth the extra $10 you will spend on your memory card to not have problems with your card. While 99% of the time the other brands will work fine, there is always the chance that you could run into errors with a manufacturer that doesn't produce as good quality memory cards. Nobody wants to have a memory card fail at the end of a wedding shoot or on the last day of a cruise when you've already taken all your photos!
And finally, be sure to buy from a reputable dealer (B&H, Adorama, Amazon – not an Amazon subseller) because there are so many counterfeit memory cards. A representative from SanDisk recently said that 10% of memory cards advertised as “SanDisk” memory cards out there are actually counterfeit. If you don’t get the real thing, you won’t get the speed that’s advertised.