When a noun is singular use a or an. ('An' is for nouns starting with a vowl) A boat floats... An elephant eats.... I ran in front of a bus. Save one for when you want to stress the number of objects. "There was only one pie left" - rather than - "There was only a pie left."

Most of the time you would use "a <noun>". I bought a car the other day. I have a daughter. You would use "one <noun>" when it is important to know the number. "How many children do you have?" "I have one child - a girl." "How many colleges did you attend?" "I attended just one college - Ohio State college."

this is an interesting question.I have the same doubt.