EEEEH! Mine looks nothing like that - mine looks like a strangled octopus!
Welcome to the world of UV mapping!
UV mapping can be one of the most complex issues you’ll face when modelling. There are SO many ways of “getting the job done” that it becomes an art unto itself aside from the actual modelling.
Many people start with a primitive (which maps very cleanly) and then deform the primitive to create the model. Unfortunately, this only works as long as you don’t add/remove a single vertex. Doing so destroys the mapping.
Most people map their model using material domains after the model is finished. This allows you to work on discrete sections of a mesh without having to untangle the rat’s nest presented by mapping the entire model.
Most folks here use a 3rd party app for mapping (I use UVMapper pro, but there’s a free version available), while others, such as Mjc mentioned, prefer programs like Blender. I’ve seen a few other dedicated mappers mentioned often in these threads (and will probably get mentioned in this one) that get rave reviews by the users.
I find Hexagon’s unwrapping tool difficult to decipher, but that could be because I’m used to using UVMapper. UVMapper also let’s you save texture templates in exquisite detail while Hexagon’s templates are just plain cheesy.
Hexagon does use seams in it’s unwrapping scheme, and another tool called “pinning”. I believe Roygee in a past thread mentioned that he figured out how to use Hex’s unwrapping tool using these pins to achieve a satisfactory layout.
Bottom line is that there’s no simple one-size-fits-all magic bullet answer we can give you on how to map your shirt.
Just take a deep breath and relax because this could be a wild ride, but when the smoke clears you can end up with a nicely formatted UV map.