While it's usually possible to create a 3D character based on any screenshot or drawing, good reference images make it a lot easier to get accurate results. Straight front and side views eliminate potential confusion with perspective, and a top view can help especially when modeling horns. Also make sure that colors are neutral and not tinted by lighting. Since characters and equipment are modeled separately, please provide references both with and without clothes where necessary.
Also include more natural portrait views of the face...
... and the full character.
For Guild Wars 2 characters, prefer taking pictures during daytime and on a map that doesn't have tinted light or a strong bloom (soft glow) effect. Your home instance or Lion's Arch should give good results. If you have the Mistlock Sanctuary Passkey, I've found it an excellent place for reference screenshots, and you never need to worry about the day-night-cycle.
You can toggle the game UI by pressing Ctrl + Shift + H. The camera sliders in the options window give you more control: The Field of View slider lets you zoom in even closer, Horizontal Position shifts the camera left and right, and the Vertical Position controls the height of the camera when zoomed in (near) or out (far). The Adjust Camera to Character Height checkbox can sometimes make a difference too.
A few things to avoid when taking reference screenshots:
Don't zoom out too far - your character should fill most of the camera frame
Avoid distracting or low-contrast backgrounds
It's not required but recording a quick video can be helpful too, especially when your character has complex fur patterns - simply rotate and zoom slowly around your character. You can even play with different poses and emotes to show different angles.
I hope this helps! Don't worry too much about "getting it wrong" - if there's a problem we can always figure things out later.