You're new to RPGs and you're looking for the sign that says "New GM Orientation"… but there isn't one. Most of us learn by doing, or by playing with a more experienced GM. There isn't really any beginner's bible. We do have some already-answered questions around here that should give you some context for your experiments, though.
Consider if you actually want to GM right away or if you'd rather learn by playing in someone else's game. (Or if you're open to both.) That will affect how you approach the rest of the process.
Find a group. You need one of those either way. It can be online or in person, but it's easier to learn the craft of GMing in person, whether learning by doing or learning by playing in someone else's game. You might have to explain what roleplaying is if you're pitching it as a new activity to friends, so reading up on that might be good. There are some good examples of what roleplaying in D&D is like as well.
If you're GMing, figure out how to start the actual game. There are a few different ways you can start the game. Eventually you'll learn that how much and what you want to prepare before the first adventure is just a matter of your taste and GMing style, but until then you have to just get started and see how it goes.
This is just a very high-level overview of the process. Follow those links to the detailed sub-steps of each of those, with their own variety of decisions and possible different paths you can take.
From there it's all a matter of growing in skill and techniques through experience. GMing a game has a lot of bits and pieces to it – plot, developing situations, improvisation, acting, crunching math for NPC stats – and it can feel overwhelming. Greg Stolze has written a good walkthrough of the things that a GM does, putting them in context, giving advice, and presenting them in a not-overwhelming way and order, so that the bits all sort of fit together. It's called How to Run Roleplaying Games, and I highly recommend it. It's not the only way to understand GMing, but it's a good summary of the process of GMing in a mere 13 pages, and gives some things to try. It's worth a look.
The Basic Rules are a place for you to start.
The Dungeon Master's D&D Basic Rules has a section on encounter building and modifying encounters (p.165-167) which will get you the same advice @Marius referenced in the DMG--at zero cost. Pay particular attention to "Party Size" on p. 166.
(The adventure contained in the Starter Set--The Lost Mine of Phandelver--does not contain notes on modifying it's encounters' levels.)
Note that the Basic Rules were revised in 2018. If you have the old Basic Rules, the referenced sections are on pages 56-58.
Next steps...
Take a look at Black Streams Solo Heroes, which has a lot of good advice for modifying adventures/campaigns/gm-style to suit the one-player experience.
Likewise, rpg.net has a good series of columns on Role-playing Duets that might give you some ideas.
But the best reference...
is to be up-front and honest with your players. Let them know you're not sure how things'll go and that you're very interested in their feedback. Some games are fun to play at the edge of survival, some players only want to play heroes who will embody the monomyth.
Most importantly:
All of the references above are distilled wisdom of other GMs over the years, wisdom borne of experience. It's all good, but none of it's going to be better than the experience you all get playing together and discussing what you like and what you hate. Use these as references for ideas, but use your players and yourself as your guide.
Best Answer
If you don't mind playing the most recent edition ...
You can do what a lot of people have done over the past 5 years, and get the D&D Starter set, or the more recently released Essentials Set
Some basics on introduction ...
The starter set has enough material to get you started. A key to doing this successfully is to discover the game together as a family. Long before this edition, which is built to be more newbie friendly than the edition I had when my kids expressed an interest, my kids enjoyed learning the game together.
We played with a light touch. And that's the key to introducing this game: the emphasis on fun. We had a session at a hotel lobby a few years later, where my younger brother, two nieces, and a nephew joined us. Newbies all. (Granted, earlier edition, but I was still DM). It went very well. Why? DM kept a light touch, and erred in favor of the players.
DM advice; keep play (turn taking) moving and when in doubt, err in favor of the players.
My wife joined for a few sessions but eventually decided it was not her cup of tea. That's also fine: forcing it is a poor strategy with a family member.
Sine you'll all be new, it may be fun for the kids to take turns being the Dungeon Master after a few sessions of getting used to the game. You'll need to see how that goes after the first session or two.