Smoothies are pretty much whatever you want them to be.
You need a liquid base: water, milk, juice
You need a solid ingredient to add texture and flavor: usually fruit, sometimes veg
Then you can add other optional ingredients for additional nutrition, flavor, texture, etc.:
Seeds, nuts, fiber, protein powder, yogurt, silken tofu, oil, bran, flax, coconut ... the list goes on and on and on.
Put everything in a blender and blend it until it's milkshake consistency. Some people add ice cubes to make it slushier. Sometimes I do and sometimes I don't.
Experiment to find the types of flavors and textures that you like the best. My usual smoothie blend is frozen blueberries, a little water, protein powder, and flax seed. Sometimes I throw in some chopped baby spinach for additional nutrition.
Edited - here's a site with a TON of smoothie recipes. Maybe you can find something here to use as a guideline or starting point.
http://allrecipes.com/Recipes/Drinks/Smoothies/Main.aspx
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