When I call someone on my Iphone 5, they cannot hear me. What do I do

take it back to the person that had it to hook it up to their itunes so they can release ownership

Your iphone may of been manufactered incorectly apple has a prograsm and they will replace it for free check out this link https://www.apple.com/support/iphone5-sleepwakebutton/

take it off mute or set the ring or volume

Try opening up the lower screen and turning off the airplane mode!

You can use Find My iPhone app. iPhone's iOS6 software now includes the Find My iPhone. Use someone else's phone to download the app and follow the directions. (Hopefully your phone has internet access and is still on :))

I paid $199 in the past for an exchange. The Apple Store is really good. If they can fix it easily they usually will for free. Back up your phone on Itunes before you take it in though, just in-case they need to reset your phone for you.

I have written up a useful guide on how to fix the iphone 6 can't hear caller problem iPhone 6 Can Hear Caller Fix

This sounds like the earpiece has gone, you should be able to hear them if you put it on loud speaker or if you are using bluetooth. However as it is only 5 months old it is covered under warranty so my advice would be to take it back to where you bought it or contact apple direct. They will repair it for you however you will be without a phone for a number of weeks