Minecraft costs money to buy, a free version is called an Alt, or an Alternative account. Once minecraft is bought you can search Minecraft in the search bar of your computer and click the icon, then the launcher will open.
Once the launher is open, you log in with your account details such as password and account name (often referred to as ign - in game name) then you’ll have to change versions as Manacube uses 1.12 which isnt the most recent update, 1.13.2 is.
It seems very complicated but trust me its easy to adjust to!
I suggest just getting to know minecraft itself before joinging Manacube as it’s not necessarily ideal for very new players to Minecraft as it has extra features & is just generally a good idea to know the basics. Hope this was helpful!