Ok, the first thing you'll want to do is in your project click on the layout editor (Just like if you were going to add a new sprite to the project) and add the AJAX object and the Array object.
Then go to the bottom right of the screen and in the Projects panel right click on Files then click on Import Files. Import the .json file to the project.
Then you'll need to add some events to your eventsheet like this:
System, On start of layout -> AJAX Request yourfilename.json (tag "sentences")
In another event under it add this:
AJAX, On "sentences" completed -> Array, Load from JSON string AJAX.LastData
Now your array is set up and ready to use. To choose a random sentence to display, you'll use something like this:
Add the Text object to your level layout (the same way you would add a new sprite). Resize the text object so it's large enough to display your sentences, then add the following to your level eventsheet.
System, On start of layout -> Text, Set Text to Array.At(0,(random(100))
The 100 is the number of sentences that you have setup.
Try that out and once you have it set up if it doesn't work then I'll help you tweak it.