How to move music on to my iphone with out it erasing everything?

I recently got an iPhone and have been figuring my way around. I had I tunes all set up on my computer, but recently that laptop was stolen. Right now I'm having to borrow laptops here and there. I want to download a new CD on to it. I transferred the music to my computer, and it is even in the itunes, but it appears that it will not let me sync my phone to it with out erasing what I already have on there. I'm signed in to my apple account, but that doesn't seem to matter.

Some one said to change the setting to manual manage music, but i can't find it, and heard that doesn't really work.

Is there anyway to put a few songs on it with out losing everything?

There's one program that allows you to do that, but I don't think it's gonna work for you since you posted that you are borrowing laptops from friends.
The program is called Media Monkey, it works only on Windows, and you also need to have iTune install in the computer.
With Media Monkey it sync the music that you want without loosing any of the music already store in your iDevice.

"Some one said to change the setting to manual manage music, but i can't find it, and heard that doesn't really work"

Click on the iPhone tab on the side of the iTunes window. In the main iTunes window, you should see a summary of your iPhone. Scroll down that window and you should be able to see the option to manually manage the music and video on your iPod. I don't know what "doesn't really work" means - either it does or doesn't. It HAS worked for me.

once you get your new computer, ainstall iTunes on it and authorize it. You should then be able to transfer to it all of your iTunes purchases.

The simplest way to save at least most of your music is to regularly save it to CDR (or DVDR if you've got more than a few GB.