There's plenty of guide on how to optimize character for most damage or utility but very few on to write a background story. Playing a rogue wearing dark cloth who is an orphan is boring.

Even orphan had parents who died for a reason, had jobs, relatives, you get adoptives parents,... but let say you do have parents, are they farmers, merchants or minor nobility.
A son of farmers may go an adventure to get coins to help his family, he may wear the chain mail of his grandfather who was in a war, and spent a week cleaning the rust from that armor, the son of a noble may be the third child with no inheritance coming so he turn to adventure, but a brand new armor his family paid for.

Or his parents dies and he suspect his elder brother to have murders them. The farmer son may have a brother who was conscripted in some Lord's war and never seen again, is he dead? captured and sold as slave? turn mercenary and don't care about his family?

A character should have a past, a present and a future goal.

A wizard usually have a teacher or mentor who taught you the basic. A cleric should have a temple local or not where she learned. Even urchin growing up in the street have gang they ran with, contacts like fence. All kind of informations that make you character alive, and open way to role play for you and the DM or more interesting plot hook.

- Your younger sister tell you that the farm where you grow was attack by goblins and your family need help
- A mentor or teacher ask your help, (or is missing) is better then just some random wizard you never hear from before.
- The assassin you beat in battle, offer information on your brother and the dead of your parents if you let him go.

What you should not do, with a level 1 character is tell how you were captain of the King's honor guard, the greatest swordman of the continent or a god who lost his powers, you are level 1 barely a notch over normal peoples.

The world is as wide as your imagination, let it go wild and have fun

The common mistake when building a first campaign is to start too big. You are full of ideas, write pages and pages of notes that too often you never got to use.

You really only need the basic for the first adventure. First a location often a village. What do you really need to know about that village. Let's call our village Douland.

In Douland you can find a mill near the river, a small inn where the farmers go get a drink after a day of work with a few rooms for the travelers, a blacksmith to make farming tools and horseshoes, a general store and a small church.

This village as a elected mayor, but is owned by Lord living in the city. This Lord as a small keep built by his grandfather but as been empty for over 30 years.

Now you adds a few names and detail. The inn is the Laughing Pig, the general store the Silver Heart, the mayor is name Alfred Burnytooth, the Lord Stephan Swooland, the church's God is Pelor.

Next you need a dungeon, since we have an empty keep already, we can find crypts below it where the Lord's relatives were buried. A good start for an evil cleric wanting to rise an army of undead in secret. Puts a few skeletons and zombies for the PCs to destroy maybe a ghoul or two.

If you have a player with noble background, make him be the Lord third son not the heir but his father can let him have the keep to own and rebuild. Now the player and his friends have a reason to visit the keep. They also have a reason to stay, and hooks for more adventures. Goblins are stealing the village sheeps? Of course the villagers will go ask the new keep owner to deal with it, they are already paying taxes to his father after all.

Add a few secrets like why as the Lord stay away from his keep, maybe his mother (the PC grandmother) died in the keep, maybe her ghost can help the group when they fix the undead problem. Finding what happened to her can be another adventure.

That's it for now :-)

Many of us started role playing games that way, we would enter a dungeons/ruins/castle for some reason, explore rooms after rooms, killing monsters along the way for loots. Then go back to town, sell the loots for gold and buy nice gear for our characters so we can do it again.

It was the way most of D&D adventure were created 30 years ago, many new players coming from computer game think like that too, and there's nothing wrong if that's the kind of game you want to play. I have been running a conversion to 5e of the original Temple of Elemental Evil and the players seem to be enjoying that kind of large dungeons adventure.

But monsters shouldnt be static, waiting in their room for the players to open the door, maybe they hear the players fighting and have ready actions to attack the first to open that door, or they can come out of their room to help defeat the group. In some corridor of the temple going west, the group (mostly level 3 at the time) opened a door to 3 ghouls, not really difficult for group working together but by round 3 more ghouls joined the fight, then more at round 7, then a pair of ghast took another round came from the north corridor getting behind the spellcasters. What would have been a serie of 4 or 5 easy almost boring fight became an epic battle. (despite my best try i didnt kill any players :-D )

Players go on rampage for a day in a dungeon, go back to the village to rest... have the dungeon owner add reinforcemnt and traps to wait for them to come back. Have monsters be on alert or patrol. A dungeon should evolve, the evil priest see that some annoying heroes destroyed his undead, have him raise more to replace them, the villain should be smart, he doesnt want the group to win after all and kill him.

P.S.: I do often threaten my players of having them kill, but all survive in the end for some reason.