It depends a bit on how tightly they are fixed, but a chisel and a small 2kg sledgehammer usually work well. An iron bar can also work well to knock off tiles. Make sure to wear full-coverage clothing, safety glasses, and gloves! The shards that fly can be VERY sharp!
Once you've removed everything, a concrete grinder is usually the best option to get rid of the old adhesive that the tiles were fixed with. If you're lucky, you might be able to scrape it off if it's poorly attached. If the concrete underneath is very uneven, you may need to use self-leveling compound before you can lay parquet on it.
(Consider also whether to prepare for water-based underfloor heating at the same time!)
Once you've removed everything, a concrete grinder is usually the best option to get rid of the old adhesive that the tiles were fixed with. If you're lucky, you might be able to scrape it off if it's poorly attached. If the concrete underneath is very uneven, you may need to use self-leveling compound before you can lay parquet on it.
(Consider also whether to prepare for water-based underfloor heating at the same time!)