Are you wanting the shirt to be a conforming clothing figure, such as common to P4 or a dynamic clothing figure for use in P5? If a conforming clothing figure. Then the mesh has to be sectioned into the proper body groups and a cr2 made up that has the correct jp settings for the figure you want to use the shirt on. Often you can rob an existing cr2 for a similar item and change the figureResFile path so that it points to the location of your object geometry. In P5 you can import the mesh into the clothroom and convert it to dynamic clothing by running it through the similator. This is not conforming clothing, but the mesh will conform to fit your figure (or should). To me more specific, it would help to know what exactly you are wanting to do.