Try starting with this base recipie: 1 cup orange juice, a tablespoon of honey, two heaping tablespoons of yogurt, half a banana, and two different types of frozen fruits. Load all of your room temperature and liquid items first. This means yogurt, banana, protein powder, juice, whatever. Quickly blend that, and then start adding your frozen stuff. This way you can add frozen things to approach a frozen consistency, rather than having the entire blender freeze up on you and having to add more warm items to liquify it enough to mix well. This way, you also never have to mix the smoothie, like in the shops you see them picking the blenders up, smacking and shaking them around like maracas.
This recipie should make about 3 fluid cups, as marked on the side of your blender. With some experience, you can more accurately add frozen fruits by checking the markings on the side of your blender. I can usually get 90% of the way to the right frozen consistency, and add the last few fruits to get it just right.
Now that you understand the basic process, I'd recommend starting with a bag of mixed frozen fruits, plain yogurt, bananas, orange juice (small single-serving containers work GREAT for storing stuff at work). This is enough material to make half-a-dozen smoothies, and with the mixed bag of frozen fruits, you have enough variety to get started with your experimentation; choose different fruits, add more kinds of fruits, add fewer kinds of fruits. If it doesn't turn out, you've only spent about two to five minutes making it, so you can easily just pour it down the drain and start over. I did this many times, but the most memorable was the coffee flavoured yogurt. That just didn't work for me, though it may for you!
I have found that some ingredients have a milder flavour than others, and since the stronger flavours pretty much mask the others completely, I have started referring to them as "filler". I use them to add bulk without affecting the flavour. So, here's my personal opinion on what's strong, and what's not:
I recommend making your smoothies with at least four ingredients, preferably more, of which at least two of which are strong. The two strong flavours provide contrast with each other, and the rest fill in and bind the taste together. Bananas and yogurt do very well to bind the smoothie together. If you just put a bunch of fruit in a blender, and just use water to keep it liquid, it will taste just like that - water plus fruit.
I would stay away from using fruit juice concentrate, as they are far too sweet to be used in a smoothie, plus it makes the smoothie taste like juice-from-concentrate. Natural sweetness is far easier and better tasting. If your smoothies aren't sweet enough, use honey.