My children play Minecraft based on extensive "lore" from friends and experimentation.
Of course, one can find anything one wants about it by googling, but if discovery is supposed to be part of the game, what knowledge is one officially supposed to have as one starts playing Minecraft?
There is no right or wrong way to play Minecraft.
When you first play the game you are thrown into a world with no idea of what you are supposed to be doing or how to do it. The lack of a tutorial (in the PC version) means that players usually rely on the official Minecraft wiki to learn new crafting recipes or play techniques.
The longer you play the game, the more your knowledge grows, digging deeper into the mechanics of the game (and the ground). The game is constantly being updated, so you are always learning new things.
The only knowledge really needed to play the game is how to use the controls (FPS controls on a PC, or how to use a game controller), but these are easily learnt and will feel natural very quickly.
If your children are having trouble playing the game, I would recommend this beginners guide, or let them play around in creative mode.
There is nothing that officially you need to know about this game before you can start. You can begin and go discovering all the details and forging your knowledge about the game while you play.
Maybe you will need a few deaths to learn some basics, and then you will be fine. It's a game about experimenting. You can form your knowledge using the wiki and the Internet when you need something specific, but I don't think there is a need to start with basic knowledge.
One of the things I most enjoyed about the game, in the early phases (alpha) was to discover all the mechanics and things the game can do. Specially if you share that with someone.
However, there is a useful tutorial with the game where you can learn the basics, and some advanced things.
Minecraft is designed with two basic modes or play-styles.
Creative mode operates as a virtual sandbox, opening up access to all resources for experimentation, building and creativity. You can play mob-free and even flying is permitted.
Survival mode is difficult without any prior knowledge. There is an underlying plot, but it is not driven by the plot. There is way more to the game than surviving on a hilltop, and I am not convinced that anyone would be able to discover the underlying plot without helpful hints and crafting recipes found online.
When starting a survival game, know that there are day and night phases, you start at the beginning of the day, and must begin preparing for night, as mobs (monsters) can generate anywhere nearby you where there is darkness and will attack you immediately. Survive by gathering things like dirt or wood that when placed on the ground, will help isolate you from them, spend the next day upgrading your shelter, tools and gear (crafting recipes will help, but if you have the time and the will you can experiment by opening the inventory menu and dragging items you've gathered into the four crafting blocks next to the avatar of your character.
Spoiler
A couple of recommendations: when it comes to crafting tools and complex items, you will have to first find out how to make and use a crafting table. Once you have a shelter location picked out, locate and kill some sheep for wool and combine it on the crafting table with wood for a bed - once you sleep at night, you will skip the night altogether, mobs will not spawn or attack, and if you die you will re-spawn next to it.
As has been mentioned before, the PC version of MineCraft has no tutorial. The entire world filled to the brim with so many different blocks can be very overwhelming to a beginner.
Moreover, there is no real way to "guess" your way to more than a handful of the recipes, especially when you don't know what items you are able to craft in the first place. There is no built-in way of "learning" new crafting recipes, you have to simply "know" them ahead of time; this is where The MineCraft Wiki comes in handy.
I would really recommend starting with the How to Survive and Thrive series created by Paul Soares Jr. It follows a great logical progression, and takes you from making your first shelter to survive the night all the way to defeating the Ender Dragon (and it doesn't end there).
Here is episode 1 of How to Survive and Thrive. Start there and watch a few episodes (perhaps following along in your own world) until you are comfortable enough getting around and surviving in MineCraft; after that you can decide whether you want to continue watching the series, or explore the rest of the world on your own.
Personally, I really enjoyed that when I first played Minecraft, I tried to look nothing up and just guess at crafting recipes until I could figure out what they were. The experimentation was fun, and it was really cool to discover a new recipe. That being said:
This obviously isn't for everyone and definitely had some unfortunate effects, like not figuring out how to make torches for a long time. (I discovered redstone torches before normal torches) I think a lot of people would find that highly frustrating and should probably just look up the recipes as they go.
There's definitely a limit to how much you can figure out by guessing. You'll definitely miss some parts of the game if you don't look stuff up. Some recipes are completely unintuitive (you'll never make an enchantment table by just guessing, if you've never seen one before, for example), and things like nether gates. So you'll definitely want to start looking things up eventually, even if you do start with minimal knowledge.
For me the only knowledge I really needed to get going was that you break stuff by clicking and holding, and not just repeatedly clicking. (Which took me an embarrassingly long time to figure out)
For Survival Mode:
You should at least know about crafting and the tools available.
Generally minecraft doesn't have a fixed objective besides 'survive' (hence 'survival mode'), but there are achievements which provide a general backbone as to what order you should be trying to do things in, so have a look at your achievements list as you play. As long as you stay focused only on achievements available to you and don't go looking up the entire list, you should be able to avoid spoilers.
To be honest there's no real way to both avoid spoilers and know everything you need to survive without having someone with you to tell you what to do next. The one problem with minecraft is that it's so open that it's hard to survive without having foreknowledge of crafting recipes and the disasters that could befall you.
It would be good to find yourself a playthrough and watch the first few minutes, at least until the person builds themselves some shelter. It would also be good to have a browser open on the minecraft wiki so that you can look for crafting recipes or familiarise yourself with some of the items.
For Creative Mode:
Grab blocks from the menu, place them down to build things and look them up on the wiki if you're not sure what they're supposed to do.
My way is: Step 1. Collect wood (15blocks should be enough). Step 2. Make wood planks (1wood makes 4). Step 3. Make sticks (20is good). Step 4. Make a wood pickaxe (3 planks and 2 sticks) Step 5. Mine some stone. (I recommend 16). Step 6. Make a stone pickaxe (3cobblestone and 2 sticks). Step 7. Make a furnace (8cobblestone). Step 8. Make a stone sword (2 cobblestone and a stick). Step 9. Make a stone axe (3 cobblestone and 2 sticks). Step 10. Hunt some sheep (3) Step 11. Make a bed (3 wool and 3 planks). Step 12. Sleep if it is night. Step 13. Chop some more wood. Step 14. Make the wood into planks. Step 15. Hunt some coal.(Any amount, the more the better). Step 16. Make torches (coal and stick) Step 17. Make a door (6planks) Step 18. You can now build a house to live in.
