Voy a llevar una camiseta prestent tense?

"Voy" is in the present tense. This is a very common mechanism that uses the present tense of "ir" to speak of the near future. It is used the same way in English: I am going to wear a tee shirt."

To really speak in the present tense, as in "I am wearing a tee shirt" or " I wear a tee shirt", You would conjugate the verb llevar: "llevo una camiseta".