When you use the Create > Cloth, Hair, Acc... option from the Menu Bar, your clothing will initially be an accessory, but will become cloth when you select "Transfer Skin Weights".
So the first question is do you have Character Creator 3 Pipeline? This is required to convert clothing.
If you do, then the Transfer Skin Weights option should be available from the Modify Panel > Attribute panel.
I would advise watching the tutorial below which will take you through the process step by step. If you then still have problems, please let us know at which point in the tutorial you encounter problems, and try to include a screenshot showing the issue so we can what is happening. Thanks.Character Creator 3 Tutorial - Creating Clothing from OBJ Files