When you create a new document, set your trim size in the "Page" section of the new doc dialog box. Set inch sizes as decimal numbers followed by the double quote mark (i.e., 8.5"), or follow the general Quark guidelines for using the measurement system of your choice. Page size = trim size.
In the same dialog box, but over to the right, set your margins to give you a text box equal to your live size requirements. Page size minus margin values = live size.
Set your bleed spec in the Bleed tab of the Print dialog box as so many picas or points or what have you. If you want a quarter inch bleed, enter .25". If you are using master pages, you can set ruler guides on them to help position page elements so they bleed correctly (to whatever your spec is), but producing the bleed on the output is I think a function of the print settings.
These settings can all be reset on an existing document: page size in the Layout/Layout properties... dialog, margins by selecting the master page(s) then choosing the Page/Master guides... dialog, and bleed in the print dialog.