How to sync music to my iphone 4?

I already put the songs into a playlist. How do i go from here?

Added (1). I already did that. I put the songs onto an Itunes playlist and my phone is plugged in.

Just plug it into your computer and open itunes

Connec your phone to your computer. Open iTunes. Drag and drop the songs from your playlist to your phone. Done.

"I already put the songs into a playlist. How do i go from here?"

why would you do that? I've got 450 songs on my iPod and never had to rely on playlists. Just set the iPhone to be manually managed for music and video, Then drag the songs from their current location - iTunes, the windows music folder, wherever - and drop it onto the iPhone tab on sidebar.