It's funny how things like this go crazy when you look at how different editions on different languages work.
Hit Dice, in Portuguese, was translated (incorrectly) as "Dado de Vida", or "Life Dice". In portuguese, you don't have "Hit Points", you have "Pontos de Vida", or "Life Points".
So, at least for portuguese-speaking people, associating "Life Points" with "Healing"(Gaining "more life") is trivial.
The same don't happen on english, however. I had a player who was on the same situation a few days ago, having this same issue (albeit on another system). He simply couldn't plug on his mind that the same value for "Hit Dice" would be used for Healing.
So, we went up to try to find a few mnemonic ways to remember that easily. We came up with something... fuzzy. We went to write some verses, to find something easy to remember. We came up with this (Keep in mind that I'm a GM, not a Poet, so yeah, that's a heck of a silly thing):
When Ones roll plenty, and the battle goes bad,
No fear I have, I have to say that,
One of them is a Cleric, and good one in fact,
Always prompt, ready to act
And because of him, I always remember
That to my well being, on that dreaded chamber
Dear to the Heart, My hit dice should I Keep
'Cause with them he will heal me, in time of need
(It's actually bigger than that, but I removed the non-relevant parts)
The last two verses did the trick, when you read them in a "Dr. Seuss"-like way.
Yeah, it's bad, but hey, it worked!
Hit dice have something of a misleading name. It would be better to call them something along the lines of "hit point dice".
They serve two functions in the game. First, they determine your max HP. At L1 a PC gets the maximum roll of their hit dice (so like hte fighter's hit die is a d10, so they get 10), plus their con mod (we'll use +2 for our example here). This means that a L1 fighter starts with 12 HP. Each time they level up after that they get to roll their hit die (or take the high average), and add that plus their con mod to their max HP to get their new max HP. So back to our fighter example, when he goes from L1 to L2, he gets to roll his hit die, lets say he gets a 6, which is the high average), and then add his con mod. So a L2 fighter would have 20 HP.
The other thing that hit dice are used for is for healing during a short rest. If you take a short rest following combat, you can spend (roll) hit dice and add them to your current HP to take you up to you max HP.
So yes, you are currently using your hit dice wrong. The monster stat blocks are worded like that to show you how the monster's HP are arrived at (2d6's average is 7). If you want to randomize the HP for your monsters you can roll instead of taking the average (it's the same thing for monster damage, they give you the average, you can choose to randomize it with the dice expression provided).
If you were watching someone play the first adventure of the 5e starter set, most of the monsters there are goblins with 7 HP and, yeah, most L1 PCs can one shot goblins. This is normal.
Best Answer
The DMG outlines the process for creating monsters as a DM.
There are two ways to decide how much HP your monster has:
Method 1: The CR Table
In this method, you are going to do some math to go from the hit point range of your monster to the number of hit die your creature has.
In this case you use the table "Monster Statistics by Challenge Rating" (DMG p. 274) to look at the line with your expected challenge rating and see the range of hit points in that table.
Make sure at this point you know the size of your monster because that is what determines the size of the monster's hit die.
I am going to quote this AngryGM article because it explains this part of the process well:
After going through the above process you should now have the number of hit dice your creature possesses.
Method 2: Assigning Hit Dice
This one is very easy: you simply assign the monster's number of hit dice and make sure that the CR comes out the way you want by adjusting other features of the monster further down the line.