Levelling compound, cement based, like Gnu says. Ready mixed, you just wet it. After that, it depends on what kind of tiles you install. You should get instructions on the pack. They may be self-adhesive.
Cork or vinyl, I use a spirit based adhesive, ceramic tiles need a cement based adhesive, and for ceramic over a wood floor, you need an underlay/overlay of fibrolite/cement board (heavily) nailed down, assuming that they weren't ceramic before.

Any levelling compound other than cement-based would need to be twin-pot, chemical setting.

Set the job out carefully, lay first in a straight line down the middle, then one side first, the other after, maybe leave the cut tiles around the edge until tomorrow.