Congratulations on finishing your first quilt! Since it's SITD I am assuming you machine quilted it? The 3 layers of the quilt should be in the binding. I square the edges of my quilt sandwich before adding the binding by machine to the top of the quilt and hand stitch on the back. Sometimes I want to do machine binding and then I use Charismah's tutorial on QB that is titled, "quick machine binding with flange."Some quilters like to draw a line instead on thir quilt edge and attach the binding along the drawn line and then trim.
I bought the easy thread needles from Walmart for burying my thread in the quilt sandwich. I tie a knot with the 2 thread ends and pop the threads into the top of the needle and brry the ends into the quilt batt in the center of the sandwich and trim off the extra thread.