First, Welcome to the site! It is also a great place for making trades with other members by matching wantlist and trade lists. The folks here are super friendly and helpful.
Go to the player page. Then go to Collection Summary which should be on the right side (desktop version) under "Your Collection". Then, go to "Add / Update". Then you can use the drop down to add all missing cards to your wantlist. It should be that easy. You can also go to "Add / Update Wantlist" and then use the checklist to add each card to your wantlist (in case you don't want EVERY parallel and SP and 1/1 version).
Good Luck!
-Dan