You will need a full desktop computer (a laptop could be used) to act as the server already running Debian Linux. If you have a spare computer that you do not mind losing what is on the hard drive, you could install linux on it and go from there. You need a computer network to connect the thin clients and the server (usually a network switch and cabling). You need some thin clients that support the rdp protocol. Not all thin clients do. I have some thin clients that expect to load the software to run them selves from the server when you turn them on. So if you plan to purchase thin clients you need to know these things. You will want some that have the needed software already built in. Purchasing used thin clients from the net can be a real minefield of problems. I know I bought some units of one kind and ended up with 4 good ones out of the six I bought. Sellers tend to sell them with parts missing. But I bought them so cheap I can not complain. The Compaq Evo series is what we use most. We also use some Neoware units. Both companies were bought by HP.. The Wyse unit pictured is based on the Compaq EVO series.
Note: if you have an Android or Ipad tablets with an rdp application, skip over to step3.