"Where are you going?"

I believe that it's not good Dutch to drop the naartoe, however you can use heen in its place

Same question... Anyone?

it gives the idea of direction and movement, i guess, but i'm not a native speaker

Waar ga je naartoe? and Waar ga je heen? are both fine.

I think this is better Dutch. I speak Afrikaans (which is very close to Dutch) and that's basically how we would say it.

Because we don't know where we're going yet. "er" would mean that we have an idea of where we're going. Here is an example of how this exchange could go: 1- Waar gaan jullie naartoe? 2-Wij gaan naar onze vriend toe. 3- Wanneer gaan jullie ernaartoe? 4- Wij gaan er nu naartoe!

1 - So in the first sentence, we don't know where you (and your group of friends) are going, so there's no "er". 2 - In the second sentence, you tell us where you are going : you're going toward your friend. As you've noticed, naartoe is actually two words put together : naar (which you could translate to "to") and toe (which brings the element of movement to the "to"). When you want to tell where you're going with more than just "here" or "there", you will use "naar" like you normally do, and then add "toe" at the end : ik ga naar de school toe, ik ga naar hem toe.... 3 - In the third sentence, the person asking now KNOWS where you are going/what you're going towards so they can use "er" connected to "naartoe". ernaartoe stands for "naar jullie vriend toe" in that sentence. 4 - In that fourth sentence, you can find an example about why the order of the words in a Dutch sentence matters quite a lot : in a normal affirmative sentence, "er" will go after the verb. BUT that goes ONLY for "er". If other complements (like "nu" in this sentence) or a negation are also in the sentence, it sends the "naartoe" away from the "er".

The order of words in Dutch can be a complex subject, here is an excellent explanation about it : http://www.dutchgrammar.com/en/?n=WordOrder.00 .