Drop down your console by pressing the ~ button. Make sure you add -console to your shortcut parameters to ESF before doing this though. Then type in the cvar or command you want, lets say mp_plcatchup. This cvar has two options (True or False). The numbers 0 and 1 represent True and False. So an example would be:
mp_plcatchup 1
It is basically about memorizing, or kinda guessing how they work by what they are asking. Like changelevel changes the map so obviously you would type in a map name. On the other hand mp_fraglimit would be how many kills it would take for the server to restart. So obviously it wouldn't be a map name, but a number which would follow the command after the space.
Just write them down somewhere, you will know them eventually
.
For a more detailed list by Mr. Satan check
here. Note that the first post is 1.0 Cvar's but if you scroll down to the second last post, he adds the new Cvar's for 1.1
.