* 一個 (ikko): This means "one" but is often used where English would use "a" or "an" with count nouns. For example, "a car" would be 一台の車 (ichi-dai no kuruma), using the counter だい (dai) for vehicles, but "an apple" could be 一個のりんご (ikko no ringo).
* 一 (ichi): This simply means "one," and sometimes works as a substitute when the counter is implicit or understood.
* ～の (no): This particle indicates possession or attribution, and sometimes functions similarly to the indefinite article "a" in English, particularly in sentences like "He's a doctor" which could be translated as 彼は医者です (kare wa isha desu). The "a" isn't explicitly translated but the meaning is conveyed.
Therefore, you need to consider the noun you're using to determine the best way to say "a" in Japanese. Provide a sentence, and I can give you a more precise translation.