Centering an item is not always easy. The geometry itself will have a default position in Poser space that may not be in the center of the Poser universe. Let's assume that the default position is centered. In that case all you need to do if its a prop is select the item, and zero the x, y, and z translations in the Paramiters Pallet. If its a figure you will need to zero translations for the root actor (usually "BODY"), and the base body part (usually "hip"). If that does not do the job, just yell, and we'll look at other ways cntering things. Restore Element (Ctrl+E), and Restore Figure (Shift+Ctrl+F), can also be useful here, though using these will also restore the rotations, scale, and other paramiters. You can control most things by selecting the item and using the paramiters pallet, others by from the menu bar Object > Properties, and materials from Render > Materials. A bit of experimentation with these should be enough to get the basics. After that, look at the tutorials at this site, and others, such as Poser Pros, RDNA, 3D Commune, and Nerd 3D. And if you have a specific question there is usually someone here in the forum who can help.