If this us Ubuntu you are working with, by default 'su' is disabled, to switch to root. Under Ubuntu, you use 'sudo' instead of 'su'. The sudo command gives you root privileges for one command. The password you supply is your user password. You type it, the cursor doesn't move, press enter. If successful, the command executes.
This is a difference in distros, and applies only to ubuntu ( and other ubuntu flavours. Sorry I didn't mention this earlier.
BTW, you can enable su in ubuntu, but I won't add that confusion now.
Hope this helps.