I reviewed a couple of your games and can see one of the mistakes you making is putting a melee attack monster in second position that can't attack. In most cases its better to use a card that has reach (green claw) or magic in second position.
For basic strategy: Large health melee in front / Magic in the middle / Ranged in the back. If mana allows put as many magic in your line up as possible behind the 0 Mana chicken.
The water splinter with its magic will win allot of the times in the lower league levels. I noticed you didn't have a Sea Genie yet so I sent you one. Try the water splinter with Sea Monster in front / 0 Mana Chicken / Sea Genie and some range attack in the back.
Good luck, keep at it.
Thank you very much for the letter and I am doing what you said and the truth is that I am doing very well now