Look at your reactions. Do you know where those are? You can click on the small icons on the top right of your bar across the bottom of the screen.
Or you can open your inventory and click on the spellbook option. You should notice a reactions tab. Click on that.
Now you'll see some reactions. There are two checkboxes for each of them. One is for turning the reaction off and the other is for whether or not you want them happen automatically, or if you'd prefer to be asked.
If you want to be asked about whether or not to smite with every hit, check the appropriate box. If you want to be asked about crit smites instead of getting them automatically, check the appropriate box.
Hope that helps.
