Using the basic version of Bitsy, the only thing you can do with variables is trigger some dialog.
In the "dialog" pane you can choose "conditional" and set a condition there, so when that condition is met, a certain dialog will show.
If you want more things to happen, I think the easiest way is to use Borksy to apply hacks to your game. Have a look in particular at Exit from dialog and End from dialog.
Let's say you want to have a plot appear when you find the shovel. If you apply "Exit from dialog" you can add dialog to the item shovel (it will look something like (exit "room2, 0,0") and once your avatar interacts with the item, it will be transferred to room2 (in this case, in the top left corner) which will be exactly like the room you came from, but with the plot diggable.
Let's say that you want the plot to appear once you have the shovel, and you talk to someone. Add conditional dialog to that someone's sprite, if itemShovel==1 then (exit "room2, 0,0") will transfer the avatar to the room with the diggable plot, if itemShovel==0 you could have it say something like "go look for a shovel, you'll need it!" or something.
Hope this makes sense!