The definite article is used before ordinal numbers and superlative adjectives:
This is the first time I've ridden a horse.
The 20th century has seen the bloodiest wars in the history of humankind.
Jill is the most intelligent girl in the class.
The indefinite article is used with certain numbers and fractions:
a/one hundred, a/one thousand, a/one million
one and a half kilos / a kilo and a half
a/one third
a/one quarter
a dozen
a quarter of an hour
half a minute