how do I transfer mp3 music files from my computer to my iphone

First you need to download and install iTunes on your PC. Then you need to add the MP3 Nikes to your iTunes music library. Finally you need to sync iTunes to your iPhone. The instructions do to all that are here:

Update to the latest version of iTunes - Apple Support

Import media from a computer to iTunes on PC - Apple Support

Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

i want to transfer my music on my computer to my apple i pad.

I want to transfer my music on my computer to my I pad.