* J'ai - This is the most common way to say "I have" in French. It's the contracted form of "je + ai", which means "I + have". You use this form when talking about possession. For example, "J'ai un chien" (I have a dog).
* J'en ai - This form is used when referring to a quantity of something you have. For example, "J'en ai deux" (I have two of them).
So, the spelling of "I have" in French depends on what you are talking about.