Adjective's degree

Short adjective (1 syllable) - Comparative    add -er    eg.  long-longer
                                            Superlative      add -est   eg.  longest

Comparative ending with Y take the Y out and add -ier eg. friendlier
Superlative ending with Y   take the Y out and add -iest eg. friendliest

Long adjective
- Comparative- more ... than  eg. more beautiful than
                      
- Superlative   - the most/ least  eg. the most beautiful   

Irregular adjectives/adverbs - these forms have to be learnt:

good/well                  - better than                  - the best
bad/ badly                 - worse than                 - the worst
much/many/ a lot of    - more than                  - the most
little                    - less than                    - the least
far                     - further/ farther than     - the furthest/farthest

Try these:
http://www.englisch-hilfen.de/en/exercises/adjectives1/index.php
http://www.englisch-hilfen.de/en/exercises/adjectives2/index.php
http://www.englisch-hilfen.de/en/exercises/adjectives3/index.php
http://www.ego4u.com/en/cram-up/grammar/adjectives-adverbs/adjectives/exercises
http://web2.uvcs.uvic.ca/elc/studyzone/200/grammar/csadj1.htm
http://web2.uvcs.uvic.ca/courses/elc/studyzone/330/grammar/regcom2.htm