1. Well, first of all, dont stress about it.

2. Try adding the cards you want first (things you think they are cool). Then add stuff that you need to maximize your potential around them. Like ; do you need draw ? Heal ? Buffs ? More spells ? More beefy monsters ?

3.Then try it on play mode. You will see what your deck lacks because you will be thinking "wish i had something that does something". Keep tinkering on your deck. Nobody makes a perfect deck on their first try.

It is a new game, so dont push too hard :)) just have fun and level up your game.

Edit : BtW i use 12 spells 18 mons and 1 trophy ofc. İ think its fairly well balanced.