You should use the comparative form of an adjective or adverb to compare exactly two things. You can form the comparative by adding the suffix "-er" to the modifier (for some short words) or by using the word "more" with the modifier:

Of the two designs, the architect is convinced that the city will select the more experimental one. (comparing two designs)

Now that it is March, the days are getting longer. (longer now than before)

You should use the superlative form to compare three or more things. You can form the superlative by adding the suffix "-est" to the modifier (for some short words) or by using the word "most" with the modifier:

This is definitely the smartest, wittiest, most imaginative comic strip I have ever seen. (implying that I have seen more than two)

Note: if you are not certain, you should check a dictionary to see which words take use "more" and "most" and which words take the suffixes "-er" and "-est."