hi welcome to Commands this is were you will learn a bunch of useful things about command blocks and /commands!

Some simple commands

1. /time set day (0 to 24000)
2. /kill (players name)
3. /gamemode (1,2,3,0) or (c,s,3,A)
4. /gamerule KeepInventory (true or false or on or off)

Here is a video from sethbling about commandblocks