First off, have you read Arrays for Beginners? (https://www.scirra.com/tutorials/307/ar ... -beginners). It has everything you need to know to create, populate, and retrieve values from arrays.
Second I don't follow the question as you say you're trying to copy a random name from the first array to another array. You can use the firstnames.At() to get the first name value and copy anywhere you want (even using the Random() function to get the value).
If you can post a screenshot or a better description of what you're trying to do that might help more to answer the question (if you don't find the answer on the Arrays for Beginners tutorial page).