You can't participate directly; you need to come to the regional contest. For Aachen this is the NWERC. Participation from Germany has been quite low the past years: only a couple of teams for the whole of the country. The nordic countries and The Netherlands are traditionally strong contenders.
Experience makes all the difference. Here in Holland, most universities have their own local contest, of which the best few teams go to the Dutch National Programming Championship. Each university also sends teams to the Northwestern European Regional Contest, so for teams arriving there it will most probably already be their third contest that year.
6.023 x 10 to the 23rd power alligator pears = Avocado's number