Trademarks are used to identify products. A trademark tells you about the product (usually who made the product and a certain level of quality). In order to file a trademark for clothing, you have to use your trademark to identify the clothing as belonging to the trademarked brand and not as a decoration. Trademarks that are only decorative do not identify and distinguish goods and do not act as a trademark.
A Ford logo can be used to identify a truck. But the same logo on a t-shirt does not identify a certain brand of clothing (for example, quality, country of origin, etc.). Instead, this logo is being used as a decoration to appeal to people that like the Ford brand.
The size, location, and dominance of the proposed trademark are used to determine whether the mark is a trademark or just a decoration.
While no bright-line rule exists for making a determination whether or not a mark will qualify as a trademark, generally a small, neat, and discrete word or design feature is likely to create the commercial impression of a trademark.
The best bet is to put your trademark on a tag on the clothing itself or on a string connected label.