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Usually you can use A/AN or ONE if you are talking about one thing, but in some cases we prefer ONE.
When using a countable noun, you can use either a/an or one to talk about one thing:
- He's staying for one week = He is staying for a week.
- I'll be back in one hour = I'll be back in an hour.
- One guy I know from Japan... = A guy I know from Japan...
When we want to stress the number, we always say "one".
- It will cost you one million dollars! Bwa ha ha!
If you say "a million dollars", then it sounds more casual and there is no emphasis on the number.
When you want to make it clear that you do NOT mean two or more, you should use one.
- I was out of the room for just one minute.
- Would you like one bottle or two?
- He has one brother and two sisters.
We use one in the pattern "one... other/another":
- This is a picture of two birds. One is the male and the other is the female.
- He spent his twenties going from one job to another.
We also use one in phrases like "one day" and "one morning" etc. when we mean a particular time, but we don't know or it doesn't matter when.
- One summer, I went to France for a holiday.
- We'll go there one day.
