First of all, hello! I believe you've written this post originally in Spanish, however, I do not speak Spanish well, so I hope this post will be clear for you! My apologies for my ignorance in such a beautiful language!
My advice to a new player is to choose one or two sets of cards (example the Light Cards or Death Cards) over and over. See what card combinations work for you. Usually you want the Melee, then magic if possible and end with the ranged (archers) at the back (because imagine it as bows reaching from the back side of the battle over the rest so they can be cards played at the back).
As for buying cards: Just build your confidence with the cards you have at first. Play every day to win the daily quest rewards. Do not stress about getting up high into silver league. You can win in the lower leagues.
As for renting cards: Don't be afraid to rent cards! You will need a level 3 or level 4 summoner card first to rent in order to get your other monsters' levels high enough.
As for the market: Well, I'm really new to this, so my guidance (please remember none of this is financial advice) is not going to be too great, but try to watch the numbers and hold STEADY. What I mean is, buy and sell, sure, but don't get out of the game. This game WILL continue to grow, so just be patient with yourself and you'll find a way to win for sure!
I thank you for asking this question, and I hope you don't mind, but I believe I'm going to turn this into a blog post, because perhaps you're not the only one who is wondering as a new player. I hope this helped! Have a wonderful day and again thank you for reading my post and commenting!!
Hello yes, I have written it in Spanish, I don't speak English either (I usually rely on translators) so I'm sorry for you too.
I thank you for taking the time to explain these details, I will take your advice and as soon as I have the opportunity I will get into the game.
Again, thank you very much for your input, it is very valuable!
You are so welcome! And thank you as well! :)