Material Girl--
Sensible dieting takes a bit of math:
As a general rule you need to eat 15 calories a day for every pound of body weight to maintain body weight.
If you weighed 100 pounds (and were reasonably active) you would need 1500 calories--or five avocados--to maintain your weight.
If you want to lose weight, you should eat fewer than 15 calories a pound so that your body is forced to burn fat for your energy needs.
This is an excellent site for counting calories:
http://www.thecaloriecounter.com/
To get the caloric value for your smoothie, you'll need to look up the calorie count for each of the ingredients.