Capitalizing words in German is, for the most part, easier than English. In German, all nouns are capitalized, and most pronouns (except for the formal and "royal we" cases) are written in lower case. Unlike English, most German adjectives (including nationality) are written lower case.
Der Unterschied zwischen deutschen Texten und englischen Texten...
The difference between German lyrics and English lyrics...
Caption 34, Frida Gold - InterviewPlay Caption
Nor are adjectives capitalized, unlike the English title case in headlines or names of films, songs, etc. For book and film titles, only the first word and nouns (or nominative cases) are in upper case.
Deswegen gucken wir jetzt einfach mal rein in
Therefore we'll now simply take a look at
„Das heimliche Geräusch“.
"The Secret Noise."
Caption 10, Kurzfilm-Festival - Shorts at MoonlightPlay Caption
The only exceptions are if the adjective is part of a proper name, such as of a species, a legal or historical term, or a place name, or titles of books, films, etc.
Nach dem Zweiten Weltkrieg wurde Berlin in vier Sektoren unterteilt.
After the Second World War, Berlin was divided into four sectors.
Caption 1, Berlin - der alte amerikanische SektorPlay Caption
Nominalized adjectives are adjectives that are used as nouns, and in German these too are written in upper case. Generally, an adjective that has the definite article before it (der, die oder das) is a nominalized adjective:
Das ist das Beste, was es gibt auf der Welt
That's the best thing that there is in the world
Caption 36, Monsters of Liedermaching - Ein PferdPlay Caption
A possibly confusing exception are superlative adjectives, usually preceded by am and written in lower case:
Was hat dir am besten gefallen?
What did you like best?
Caption 33, Umweltlernen - Propellerpflanzen am KräutertagPlay Caption
Sometimes too, adjectives are written apart from the noun they are modifying and may at first appear to be nominalized. As you see in the following, schönsten actually modifies the preceding noun Auswärtssiege:
Am Samstag, da wir eh alle nach Kaiserslautern fahre
On Saturday, since we are all already driving to Kaiserslautern
und Auswärtssiege die schönsten sind,
and away wins are the most beautiful,
muss [sic, müssen] auf jeden Fall drei Punkte her.
three points are definitely a must.
Captions 54-56, Fußball - Eintracht-Fan gewinnt WimpelPlay Caption
Find some nominalized adjectives from this list, then search for them on Yabla German to see them used in a real world context.