First apply for dorm (
I am giving some hints which will help you to fill up the dorm application.

You can see your waiting list after applying 

Another very important thing is that, you will get an email every month to update your desired date for the dorm. You have to update it within a given period of time. If you forget then your whole application will be deleted automatically.

It usually take a long time to get dorm as there are lots of international students in RWTH Aachen.
It might take 1 semester to 1 year.
Mind it , before coming to Germany you have to manage any accommodation  of your own otherwise you can’t do anything like city registration,opening bank account etc.

So start looking for private apartment which is also very competitive at aachen.
use this website to look for private WG(apartment). Usually it is difficult to get accommodation before coming here. But you have to try. One thing to remember, there are lots of fraud deal so before sending any money talk with current students. Use email alter for this website to get alert for your required WG. Get used to this website by spending more time.

2.  This FB group is a very helpful for students at Aachen. Here you will find short time accommodation at dorm . You can get 1/2/3months contract at dorm from this group. If you don’t find any accommodation , it is the best option to get 1/2/3months contract for a room initially. Always look at this group posts.

3.  Also check this group

For media informatics students, they recommend to stay first semester at Boon .But you can also stay at Aachen.My advice is to stay at aachen as there are lots of part time job opportunities at aachen rather than Bonn.  For that i recommend to talk with current students.

I have written this within very short time. 
If you have any question ask in the group. 

Many thanks
SSE, RWTH Aachen