Knowing which skills to apply to which tasks with an uncertain outcome the players are having their characters undertake is important, but it is not as important as understanding the basic conversation of the game (Basic Rules, page 3) in my view. That being:
1. The DM describes the environment.
2. The players describe what they want to do.
3. The DM narrates the result of the adventurers' actions.
Somewhere between 2 and 3, the DM must consider whether the outcome of what the players want to do is uncertain. Only then does the DM call the rules of the game into play to resolve that uncertainty. For example:
Does the lie the character tells work on the NPC? If the DM thinks "yes" or "no," then just narrate the result. If the DM thinks "I'm not sure," ask for an ability check - Charisma (Deception) perhaps - and stick by the result.
Does the character's attack against the defending hobgoblin succeed? If the DM thinks "yes" or "no," then just narrate the result. If the DM thinks, "I'm not sure," ask for an attack roll and stick by the result.
Does the jet of flame scorch the character? If the DM thinks "yes" or "no," then just narrate the result. If the DM thinks, "I'm not sure," ask for a saving throw and stick by the result.
I have written
a guide to adjudication actions in D&D 5e. Take a gander at it if you have the time and desire. It contains a lot of examples of play that may be of help to you.