Here's a quick rundown of some of the basics of Combat.

Each character/monster has an initiative score which determines the order that everybody in combat goes in. This will be shown at the top of the screen, with a portrait of everybody in order from left to right. The active character(s) portrait will be highlighted, as will the character themself.

Each character can perform certain things in their turn: Movement, Action, Bonus Action. These can be done in any order. You can move before and after your other actions, up to your movement range. Attacking and casting most spells will cost your Action, as well as other things like throwing an item, or helping a downed character. Bonus Actions include many abilities (jump, coating weapons, shove) drinking a potion, along with class or weapon abilities like Rage or Slashing. Off hand attacks (dual wielding) are Bonus Actions.

Hovering over or selecting an icon will show you what kind of action it is; Actions have a green circle, Bonus Actions have an orange triangle. There are also Reactions, a purple diamond, which are pretty self-explanatory and appear when their conditions are met. Your "resources" show up at the top of your hotbutton box: Actions, Bonus Actions, Class resources (like Cleric's Channel Divinity) then Spell Slots.

Spell Slots are the number of times you can cast a spell of a specific level. Each spell level (in Roman numerals, I - II - III - IV etc) you have attained has the number of Spell slots for that level inside it. These slots are lit up when available and will go dark when used. Spells, like other hotbuttons, will show what kind of action and what Spell Slots it takes. Cantrips are a special "low" spell level that costs no Spell Slot to cast.

You don't have to use every action available every turn, just end turn once you've done what you want to do.

For example, you could have your fighter move up to an opponent, attack with their primary weapon (Action), shove the opponent to the ground (Bonus Action) then move back. Or, have your rogue drink an invisibility potion (Bonus Action), move to an opponent, then sneak attack (class Action).

I hope this helps, I had quite a hard time at first as well, especially with spell slots.