DnD is wonderful and also a bit intimidating for the fact that your imagination is the limit when it comes to what is possible. For combat there are a number of actions that are most commonly used:
CAST A SPELL
USE AN ITEM
READY AN ACTION
USE AN ITEM
A combat turn is divided into these components, all of which may be taken ONCE:
Action, Movement, a single Object Interaction, and a Bonus Action if available.
Uses your Action, is rolled using d20 + (Attribute Bonus) + Proficiency Bonus (if applicable) vs, enemy AC. Damage is then rolled: d(weapon die) + (Attribute Bonus)
Cast a Spell—
The time needed to cast a spell varies (check the spell description), but is generally either an Action or a Bonus Action. Each spell will resolve its effect differently, but generally it is either a Spell Attack which is treated identically as an Attack above, or it forces the target of the spell to make a Saving Throw vs. your Spell Save Difficulty Class, or DC.
The formula for your Spell Save DC is 8 + (Attribute Bonus) + Proficiency Bonus (#NOWAY)
Brace yourself, kiddos—this is about to get a bit complicated.
To save both myself and you guys some pain, here’s a link wherein someone smarter than myself has compiled all the information FOREVER about grappling:
Uses your Acton, until the beginning of your next turn all attacks against you (that you can see) are made with Disadvantage.
Uses your Action, allows you to move out of an opponents threatened area without incurring an opportunity attack.
Uses your Action, allows you to move your move speed AGAIN.
Ready an Action—
Uses your Action, allows you to prep another action of your choice to occur during circumstances that you specify: “I prepare to shoot the first thing that comes through the door”. If in this example an ALLY were the first thing to come through the door, you may instead choose to lose the readied action.
(I have to double check how to resolve this)
(WORK IN PROGRESS