First of all, join the Make your own hoodie cult
https://www.newschoolers.com/ns/cult/forum/cat_id/5626/
Second of all, here is the basic way to do it. Your mom will be able to explain it more thoroughly, and help you if you get anything wrong.
You have the two fabric pieces on top of each other, the outside parts touching each other. When you are sewing you should see the side of the fabric you want inside. The edges which you want to sew should be flush. Then simply sew along the edge you want to join. When you are done, you will have a nice, even seam. Good Luck!