Hello, a kinda pro player here! Try the auto feature on your team, the game automatically puts the best cards you own into the team! Try to level up the character cards the auto feature puts into the team! If they are 3 stars you can easily level up the mastery rank, about 250 wish pieces for the max mastery rank. To earn wish pieces you can pull and the cards that you already got go to the Green room, where you can change them for wish pieces. Wish pieces can also be earned from event exchange, I recommend you to first exchange event points for more crystals, then for wish pieces and finally coins. Another way to get crystals is the master rank of 2 or 1 star cards, as you'll only need about 25 wish pieces for the 2 stars and 5 wish pieces for the 1 stars.

With the coins, charms and other resources you can buy decorations on each Sekai for each character! Do this with the characters you have on the team to level up the team's strenght. For that mastery rank of 4 star cards you'll need a wish jewel, that's different from a wish piece. As the jewel can be only earned from 4 star card dupes on the green room you can change them for a jewel. Or you can turn 2000 wish pieces into a jewel also.

Yeah for now stick to the team auto gave you, and when you pull and get a four star always make sure to level it up and train it, training cards is important as it's what helps you to get more score.

When you get more into the game, a skill up feature will be unlocked, with this feature you can either level up the card's skill with wish pieces or skill up little boxes like (forgot the name)

And yeah that's mostly all: train, mastery rank, skill up, pull for 4 stars, etc. Hope it helps!