Getting a command block:
To begin, in the chat, type in /give @p 137.
This will give you a command block.

Basics of the command block:
The most basic of commands you can use in the command block is /time set or /gamemode .
These commands would work the same as if you would write it into the change yourself. With these commands, you can make an adventure map or something, where if they click a button, step on a pressure plate or something, it can set them to something, or make it daytime/night time etc.

The /effect command
This command will work as it would normally if you where to type it. (/effect )
This command is very useful, since you can do a lot of things with it, lets say, if it turns nighttime, you can have a daylight sensor, and when it turns night, it gives everyone night vision for 10 or 15 minutes. (/effect @a 16 600 1)

Testfor
The testfor is much more complex than the stuff that you just read. The test for, tests for something, and if it yets a posititive, then it will send an output into a comparator, making the next command block do something, so you can make a whole chain of these. E.G: You can test for someone on creative mode, then set them onto adventure or survival. (testfor @a[m=1] /gamemode 2 @a)

Thanks for reading! This is my first blog, so if you liked it, please diamond it.

Thanks,
regirock524