The tees I use are Gildan Softstyle Ladies which is more fitted than their regular tees. They also have shorter sleeves (which are more flattering on fitted styles).
I also use Gildan Heavy Cotton tees which are big and baggy - perfect for the oversized look !
When you wash, please wash inside out and on a low heat (30) 
You should always air dry your clothes so as not to damage or shrink them, no matter the fabric.