Sir, you asked a question and two people have provided comprehensive answers as of the time that I am writing this.
Your failure to specify that you wanted a response in km does not deserve a poor rating for the respondents.
Converting miles to km is simple math. Now that I am aware of your "hidden condition", the answer you seek from my response is 9,460,730,472,581.431 km. Joseph is an equally-responsible expert here and may supplement this with his own calculations.
Consider presenting the WHOLE question so folks can respond with exactly what you want; "What is 1 light year in km?"
By the way, Simply Explained does not exist to do your school homework, but some of us will "take you to school" if you appear to require education. This is as much social interaction as it is responding to questions. You have to tell us exactly what you want and there are many standing by to help if you will allow us to do so. Be specific and not so quick to judge us as less than adequate if your question was incomplete.