Don’t say: “I took one pen and started to write.”
Say: “I took a pen and started to write.”
Use one when the number is important; when you want to emphasize that it is only one (not two or three or more):
- One of these eggs is rotten, but the others are OK.
- I wanted to buy three CDs, but I didn’t have enough money, so I bought only one.
In all other cases, when the fact of being “one” is not important, use a / an:
- I had an omelet for breakfast.
- I bought a new CD yesterday.