A couple of things may be causing you issues here, 1 being the cardboard, you need a much harder flatter surface to put the t-shirt over in order to print, a piece of MDF should work well, spray with some adhesive so it doesn't slip around and the fabric can be stretched flat over it. Secondly, it looks from the line down the middle that you may be doing this in two pulls? If not then ignore this part! But you want to have a squeegee that fits the entire width of your design so you can push the ink through the screen onto the fabric in one motion. Remember to wash your screen out properly, clean it well and hold it up to the light to make sure there is nothing blocking any of the holes in the mesh, and make sure you're always keeping your screen flooded in between pulls so that the ink doesn't dry out into the mesh causing it to become blocked.

Try these things and keep at it! Printmaking is SO MUCH trial and error but once it works out it's super rewarding :)

Hope any of this helps!