Wishing you a very warm welcome, @Eliza_01! 
From looking at the cards you drew and the question you asked (“what spell will be best for the waning moon?”) here are my personal thoughts:
I see the Eight of Wands to be about movement and change, the King of Swords to be about facing things (even dangerous things) with authority and taking your own power, and the Reversed Moon to be able seeing past illusions and overcoming fear.
As for the Waning Moon 
:
What does a waning moon symbolize? This lunar phase is ideal for works of removal and banishing spells. It symbolizes the closing of a cycle which will eventually give start to a new one. It’s a perfect moment to get rid of everything harmful and all that hurts.
From Spells8 Energy of a Waning Moon
With the energy of the Waning Moon and the cards you drew, I would personally recommend either a Banishing Spell or a Cord-Cutting Spell and Shadow Work 
These are just my personal suggestions- I always feel with tarot that you are closest to your cards and know yourself the best, so your personal interpretation should be given the most weight when making the final decision 
Whatever spellwork you choose to do, wishing you all the best! Blessed be and happy casting 