Now for your bio hair, I don't recommend shaving your head, unless it's something you really want to do. We have another great article on this, you can read: HERE. We do also have a lot of customers that still have their bio hair and wear wigs!
I personally recommend braiding you bio hair and pinning it up, so it can lay as flat as possible and wearing something like a fishnet cap under your wig. (That'll help give it some grip too!) That's going to help eliminate some of the volume on top.
A good wash can also help to flatten out a wig and leaving it to dry on a wig stand instead of upside down. (Just make sure it's a style you want to keep, we can't accept any returns or exchanges that have been washed!)