Firstly, with a polo shirt you don't need to draw the collar, since there is a shirt model with a collar (Shirt02).
The details look a bit messy. What program are you using? Again I would recommend something like Photoshop or GIMP, the latter being easy to get and free. Something that allows you to draw paths and use layers is very useful and will allow you to make outfits more easily.
Keep trying and you will improve, it took me a while to learn how make outfits well!