@flauwy is good to follow, for splinterlands advice.
You'll want to level up your summoners, as their level limits the level of monsters you can bring to battle.
Do your daily battles as soon as you can. Keep and combine all of your rewards cards.
Before you click 'Rumble' take a look at the field and play the battle out in your mind.
Look at speed, who will go first, how much damage will they do, etc.
Choose who you think will win, and how that will look.
Watch the match and see how your prediction went.
If you were wrong, watch it again and work out why you were wrong.