Notes for Thaoran:
No need to alter your post - if somethng attacks you then I will have you cast shield in response to get the big armor class bump.
Shield is a spell. As a third level eldritch knight you can cast a first level spell twice before you need a long rest to get back those spell slots. You options are shield, magic missle and comprehend languages.
Shield - I already described.
Magic Missle would take an action to cast (so no normal rapier attack). It creates 3 glowing little darts that each do 1d4+1 damage. It is a decent damage but the most important thing - they can not miss. Auto hit. (Except - there is one way to dodge a magic mimssle - use a shield spell!) But when you absolutely must hit. This is a great option.
Comp languages - seems obvious what it does.
So you can do any of these things twice between each long rest.
But you can cast as many cantrips as you want. Unlimted.
Cantrips Thaoran has available: Minor Illusion, Control Flames, Blade Ward, Mage Hand
For these the easiest thing is to simple google the name of the spell plus "5e" and read the description.
Perhaps the most interesting is
Blade Ward: You extend your hand and trace a sigil of warding in the air. Until the end of your next turn, you have resistance against bludgeoning, piercing, and slashing damage dealt by weapon attacks.
Note that it does take your action - so no attacks that turn. Otherwise it would be way overpowered. And it only last one turn. but you can do it over and over again. Cantrips are intended to be little spells that do little stuff but don't impact the game in a big way. Mage hand, for example, can maipulate objects with a spectral hand - but only 10 lb. So you can't wield weapons. But you can use it to open doors and potentially trapped chests. Cantrips are fun - check them out.