An enchanting table takes four obsidian, two diamonds and one book. The obsidian is the part that takes planning, because you need a diamond pickaxe to mine it in the first place.
The recipe
Open a crafting table and lay it out like this: the book alone in the top middle slot. Diamonds either side of the centre. Obsidian filling the entire bottom row plus the middle-centre slot. Four obsidian, two diamonds, one book.
Getting the obsidian
Obsidian forms where water meets a lava source block. Find a lava pool, pour a bucket of water over it, and mine the result with a diamond or netherite pickaxe — anything less destroys the block. Each one takes around ten seconds to break, so bring an Efficiency pickaxe if you have one.
You will want spare obsidian anyway. A nether portal needs ten blocks at minimum, so mine a full stack while you are down there.
Bookshelves and why 15 is the magic number
A bare table only offers weak enchantments. Surrounding it with bookshelves raises the level of what it can roll, and fifteen shelves is the cap — more than that does nothing at all.
Place the shelves one block away from the table with a gap of air between, at the same height or one block higher. If anything solid sits in the gap, that shelf stops counting. A common working layout is a two-high ring of shelves around the table with a one-block doorway to walk in through.
What the table cannot give you
Treasure enchantments never appear at a table no matter how many shelves you have. Mending, Frost Walker, Soul Speed, Swift Sneak, Wind Burst and both curses come only from loot chests, villager trades, fishing or piglin bartering. If you are chasing Mending specifically, a librarian villager is far faster than any amount of table rolling.
Spending your levels well
Enchanting is only half the job — combining what you roll is the other half, and that is where levels get wasted. Plan the full set with the enchantment calculator before you start, and browse what is actually worth having in the enchantment list.