What is so hard to get? I'm the same age as you and I found this game to be very simple.
In battle the number on the lower right of the cards indicate what level the card is. If your enemy is using a level 9 card and you attack using a level 6 card it would result to a "card break" in the enemy's favor. When you get card break it means your attack is interupted or will not work. The higher the level card, the less chance you will get card break from the enemy. Zero cards are special, it can card break any card/sleight no matter what level but it must be played after your enemy's attack because a zero card could be crad breaked by any level card.
As for opening doors. To open a door you must use a card or cards that fit the criteria. For example, if there is a door saying "5(arrow pointing up)" and is red means to open that door you must use a red card level 5 or higher.
If there is a door that has no color then you can use any card.
If there is a door has a number with no arrow next to it. This indicates you must use a card equivalent to the number, no higher and no lower.
Does that answer your question. If it doesn't sorry, I wasn't really sure what you were asking. If you need more help then don't be afraid to ask.
(BTW: In the beginning of the game they teach you all you need to know about the cards)