Just some help

Not a new player but haven't played in a very long time. Trying to figure out is there a way to look at all cards you have without picking a hero and also playmats/sleeves. Is there a way to delete old decks or do you just have to load a new deck over it?

From the deck builder you can click on the Filter button, and then toggle Show Unusable. It will then show everything you have.
There isn't really a way to delete saved decks, but you can easily overwrite it and give it a different name.