Reading/writing to data file?

So I checked the docs and I'm still bit confused on data files. How would I save the current data? How would I add data? How would I create a data file? How would I read data?

 

I'm still a bit confused. I'm trying to make a command where you can give a player a certain amount of a int on the data file. I'm trying to get the config to look like.

Code:

PlayerID1:  {
    Items: 1
},
PlayerID2: {
    Items: 0
}

How would I change if it already exists get the item value and change it?