The first step is to take a number of photographs (between 20 and 40) of your object, so that 123D Catch can assemble a 3D model. I'm using a rock that I found near my studio.
Here are some important tips:
Photographs should capture all sides of the target object, both all the way around, and top and bottom. A good approch is to take 20 objects all the way around, and then move up (pointing the camera down at the object) and take another 20
The object should not be moved at all during capture, and lighting should remain consistent. In effect, once the object is in place, you should move around it
The target object should take up most of the frame: either get close to the object or zoom in
While it is not absolutely necessary to maintain the same distance from the object all the way around, it does help the algorithm if distances are consistent
Consistent diffuse lighting all around the object works best.
Accurate focus is important - you will want to remove any out of focus shots
There are extensive tutorials available here: http://www.123dapp.com/catch/learn
So go ahead, take your pictures. If you're using the camera rig I referenced earlier, adjust the horizontal distance on the rig, and the camera zoom, so that the object fills the frame. Since the distance to the object will be consistent, you might want to focus manually.
When you're done taking pictures, save them all to a folder with a descriptive name.