As you advance, you will be given the chance to spend your hard earned coins on larger spellbooks that will hold more spells and better yet, more copies of each spell. You will probably also get some special ones as 'boss drops'.
Just equip the new deck, choose your spell combos...and don't forget to unload your old deck's Treasure cards and move to the new (you will also have a larger number of slots for these)>
I had the same questions when I first started playing. I too am fairly new. I've only been playing for a couple of months now. I'm currently a level 25 Fire School student. I have chosen Storm as my secondary school mainly because to me it seems to be a nice combination of the two schools. As stated before, as you progress in the game your given the choice of better and improved decks which hold more spell cards. Also as you progress, you will find that there are several more spell cards to be added to your deck that are not treasure cards or handed out by your teacher. The only advice I can give you is to make sure you have a health card of some sort, defensive cards to help protect you, and have cards that will help you hurt your opponent more effectively. I know it seems complicated at first but as you play more and more, you'll learn how important these steps are to progress. As any game, the further you get into the game, the harder it gets. Luckily Wizard 101 allows you to do multiple quests for each level to help build your ranking levels up.
Go ahead and learn all the spells for your primary school. Your decks have a limited number of spaces to manage, but your spellbook will grow as needed and not run out of room. This seems to be true for both learned spells and treasure cards. While a deck may hold only about a dozen treasure cards, I have put well over a hundred treasure cards into my spellbook and it just keeps getting more pages as needed.