When you get into the site, simply click on the category of clothing you want, select the specific item of clothing, choose the colour, add text (click on Add Text), add a logo (Add Graphics then click on a category), make sure everything is where you want it, then click on Place Order. If you get stuck there is help available in the How To column.
Click on the 'buy this' button next to the item of merchandise you are interested in below to go to the merchandise site, where you can add text and logos for clothing and add/design other products: