Five types here: clay, wood, paper, silica and plant based. The filters will narrow it down quickly, but the thing worth knowing before you start is that your cat gets a vote, and it's a stronger vote than most people expect.
Cats care about texture, not smell
Most cats prefer a fine, sand-like grain they can dig into properly. Coarse pellets and large granules feel wrong underfoot to a lot of them, however good the litter is in every other respect.
This matters because a cat who dislikes the litter often stops using the tray, and that gets treated as a behaviour problem when it's a substrate problem. If your cat has started going elsewhere, the litter is one of the first things to change.
Scented or unscented
Scented litter is for the benefit of the person emptying the tray. Cats smell far better than we do, and a fair number actively avoid perfumed litter. If your cat is fussy about the tray, or you're setting up for a new cat, unscented is the safer starting point.
The better answer to smell is scooping daily and a full change on schedule, not a stronger fragrance.
Clumping, non-clumping and kittens
Clumping litter usually works out cheaper over time, because you lift the soiled clumps and top up rather than replacing the lot. Non-clumping needs changing completely more often.
One important exception: very young kittens taste and eat things, and clumping litter can cause a blockage if swallowed. For a kitten under about four months, use a non-clumping litter and switch later.
Litres or kilos
You'll see both on this page, and they aren't comparable. Clay is dense, so a heavy bag isn't a big bag. Wood and paper are light, so a big bag isn't a heavy one. What matters is how many weeks it lasts, which depends on the type, how many cats you have, and how often you scoop. Judge cost by the month rather than by the bag.
Two things people get wrong
Not enough depth. Cats need to dig properly, so aim for around five to seven centimetres. A thin scatter is the most common reason a tray gets ignored.
Not enough trays. The general rule is one per cat plus a spare, in different parts of the house. If you're short, the litter trays page has them.
And when you do change type, do it gradually. Put the new tray alongside the old one for a week rather than swapping overnight, especially with a cat who's already been fussy.
On flushing
Some litters here are labelled flushable. Water companies in this country generally advise against putting cat litter down the loo, partly because of the risk to drains and partly because cat waste can carry parasites that water treatment isn't designed to remove. Bag it and bin it.