French has two words for a (or an): Un and Une.
Look at these examples:
Notice that unlike English, all nouns (words for things and people) in French are masculine or feminine.
Use Un with masculine nouns.
Use Une with feminine nouns.
Note that un, une also mean one: une maison means a house and one house.
Grammar jargon: un, une are indefinite articles (a, an are indefinite articles in English).
See how these articles behave in negative sentences: