When you are setting up your regression model, you'll have your IV's and your DV. You need to click on the categorical button at the side of the logistic regression window after putting all your variables in. When you click on the categorical button, you then drag all the categorical variables to the right hand side and select either first or last as your reference category. If you don't like the first or last category as they are, you will need to go in and recode your variables so that the category you want is first or last.
Hope this helps.