Let's say I live in the student dormitory. If I'm writing the address should I add my room number so that a parcel can be delivered into my post box?
Yes, it's usual in dorms and advisable if you want to make sure that the parcel is delivered to you and not someone else!
As already pointed out, you can put it in the same line as the street. According to the DIN standard 5008 for addressing, you should use space, double-slash, space to separate it from the house number:
Straßenname Str. 123 // Zi. 123
The alternative is to use a separate line for it, but then before the street (that means between name and street):
Straßenname Str. 123
Also note that there is no comma between postal code and city and "Str." is always capitalized if it is a separate word ("Goethestr." but "Lange Str."). The country should only be indicated on international mailings.
Yes, you should add your room number. Because a postman has no time to search your name among 500 others under same address.
Just another abbreviation: Zi. - because the exact location has to be found by local postman, not by sender's post office.
Budapester Str. 24 / Zi. 123
Depending of the address fields of the sender it is also possible to use a new line, I used both ways in my past:
-> country "Germany" only if post is sent from another country.