German adjectives know three degrees of comparison: the positive form (that you have dealt with so far), the comparative form and the superlative form.
Now, let’s take a closer look at the other two degrees:

Building the comparative form:

To build the comparative form of an adjective, follow this rule:
positive form + -er
 
Example:
schnell → schneller
fast → faster
klein → kleiner
small → smaller

Building the superlative form:

To build the superlative form, there are two possibilities:

1. The superlative is introduced with "am", then you add the ending -sten to the basic form of the adjective.
    
   Example:
   schnell → am schnellsten
   fast → fastest
   klein → am kleinsten
   small → smallest
2. You also can build the superlative form together with the definite article following this rule:
   definite article (der/die/das) + positive form + -ste
    
   Example:
   schnell → der schnellste
   fast → the fastest
   klein → der kleinste
   small → the smallest

Comparative and superlative are declined like adjectives in their basic form.