You really have two questions: "How are adjectives declinated?" and "When do you append an 'n' at the end of nouns?". I will treat these questions separately.
Adjective declination in German is difficult since it depends on the definiteness of the noun. In simple English that means that in German you would decline "attractive" differently in the three phrases "an attractive man", "the attractive man" and "attractive man", but on top of that you also need to take care of the case. That means that you need three different tables to compile a complete cheat sheet of adjective declination.
Let me first give some examples which might elucidate how you are supposed to use the cheat sheet.
Say, for example, that you need to write about an attractive man in the nominative case.. Then the following three inflections exist:
Indefinite form (with "ein" words)
Ein attraktiver Mann stolzierte die Straße entlang.
Definite form (with "der" words)
Der attraktive Mann erregt die Aufmerksamkeit der Frauen.
Unpreceded form (with no article)
Attraktiver Mann sucht eine Frau, die ihm das Herz strahlen lässt.
But in the dative case things look quite different:
Indefinite form (with "ein" words)
Einem attraktiven Mann wurde gestern aus dem kalten Wasser geholfen.
Definite form (with "der" words)
Die Frau mit dem roten Kleid gehört dem attraktiven Mann.
Unpreceded form (with no article)
Hübsche Frau von attraktivem Mann gesucht.
I copy-paste the complete declination table (or cheat sheet) that can be found in the middle of this site. The order is, from top to bottom: nominative, accusative, dative and genitive.
After "DER"-words
M F N PL
-e -e -e -en
-en -e -e -en
-en -en -en -en
-en -en -en -en
After "EIN"-words
M F N PL
-er -e -es -en
-en -e -es -en
-en -en -en -en
-en -en -en -en
Unpreceded
M F N PL
-er -e -es -e
-en -e -es -e
-em -er -em -en
-en -er -en -er
The rule for nouns are easier. In the dative case (almost) every noun gets an 'n' at the end.
There are also some irregular nouns which have further inflections, but these do not belong on a cheat sheet. Let me in any case mention some for completeness:
- You say ein Beamter, but der Beamte.
- Many masculine words which end on -ent (like "Student" and "Präsident") are irregular
- The word "Herz" is irregular