"The main use of the past subjunctive form is in counterfactual if clauses:
If I were a badger, I would choose that color.
He would let us know if he were planning to arrive late.
Note that the indicative form was can be used equally well in sentences of this type, but were is sometimes preferred especially in more formal English. According to the Random House College Dictionary, "Although the [were] subjunctive seems to be disappearing from the speech of many, its proper use is still a mark of the educated speaker." When were is used, an inverted form without if is possible (see Inversion below); this is not possible with was."
Because you used "if", either can be used. But this shirt says that the person wearing it is educated.