シャツは一枚だけある。　シャツが一枚だけある。Both are correct. But meaning is slightly different. シャツは一枚だけある sounds like it is an answer for a question like "how many shirts do you have?" If you want to mean just like "There's only one shirt" you say シャツが一枚だけある。 Even for fluent Japanese speakers, the problem of "は and が" is so tough. I, native Japanese speaker, can use them collectly, but I don't know the rules of them. Anyway, I'm glad that many foreign people like you are learning Japanese. :)