when i make or receive calls I can't be heard

Does Siri hear you? Can you record a voice memo? If you can, it's most likely not the mic. Try a forced restart. hold down on the sleep and home buttons at the same time for about 10-20 seconds until the Apple logo appears on the screen.

You may not be doing anything wrong. You could have a hardware problem. Have you seen this?

If your voice is too faint or sounds unclear using iPhone, or iPod touch - Apple Support

I'm certainly not an expert, but the way that you have described things now, it sure sounds like a hardware issue to me. Return to the store where you bought it. Do it as soon as you can.