Adjective declension after an indefinite article

The adjective is in between the article and the noun:

Article Adjective Noun
ein rotes T-Shirt


In the nominative and the accusative cases, the adjective takes the ending of the definite article.

Colorful graphic on which the words "der Pullover" are written above "ein roter Pullover." The letter "r" in the words "der" and "einer" are each marked in blue and connected by an arrow.
null DW
Nominative  

der Pullover:

ein roter Pullover

die Hose:

eine rote Hose

das T-Shirt:

ein rotes T-Shirt

die Turnschuhe: rote Turnschuhe

 

In the accusative case, the masculine article changes form, as does the adjective ending. The other forms remain the same.

Accusative  

den Pullover:

einen roten Pullover

die Hose:

eine rote Hose

das T-Shirt:

ein rotes T-Shirt

die Turnschuhe: rote Turnschuhe

 

Ich brauche einen neuen Pullover, eine neue Hose, ein neues T-Shirt und neue Turnschuhe! 
Here is an overview of adjective declensions after an indefinite article:

 

Masculine

Feminine

Neuter

Plural

Nominative

-er

-e

-es

-e

Accusative

-en

-e

-es

-e

Dative

-en

-en

-en

-en

Genitive

-en

-en

-en

-er

 

Adjective endings following an indefinite article in the dative are simple: All adjectives have the ending -en.

In the genitive, it's almost the same except for the plural ending after an indefinite article turns into  -er.

 

Watch out:

Adjectives that end with -el or -er usually drop the  before the adjective ending:
dunkel: Die Frau trägt einen dunklen Pullover.
teuer: Der teure Pullover ist schön.

A few adjectives are not declined (superextra and some adjectives describing color)
super: Das ist ein super Kleid.
extra: Möchtest du einen extra Hut für dein Outfit?