How to make the songs and playlists the same on my iphone as on the computer?

I have songs and playlists on both my computer and iphone, but they differ slightly, and they are missing some and some have some additional playlists. I want to delete all the playlists (and songs maybe?) on my phone, and sync everything from computer exactly onto my iphone. How do I do that? I have tried several times and have failed. Also, how do I keep from songs appearing twice on my iphone?

BTW, it's itunes I'm using. And more importantly, I started downloading songs from youtube, and transfer them over to my itunes library. For some reason, some of these songs didn't even appear on my phone.

ITunes can't upload youtube songs to iPhone, because iTunes thinks the songs are not belong to your account. You may try some third party software like this iTunes alternative, it can manage your iPhone songs and playlists. You can also use it to upload files and youtube songs to your iPhone.
http://www.reviewstown.com/itunes-alternative.html
Hope it helps. Feel free to email me if you need further help.