It means when you think you are because of the other things and that’s a result of the consequences from the actions of when you decide you want to do that because of the outside factors of the unknown
In short, this means the person "going on a t-shirt" is going to die. Then, when they die, their homies will wear t-shirts with the dead person's name on it in remembrance, therefore birthing the term "going on a t-shirt"