How do I put new music on my IPhone 4g without loosing what's currently on my IPhone using ITunes on computer ?

So I've heard that if I've bought music from my iTunes store directly off my phone If I plug it in into a friends computer I will loose all the music that I bought that's currently on my phone and I've also heard that if i had put music on my iPhone from my computer and then plugged it in to a friends computer to put new music on it it would erase all the current music that on my iPhone off . I really need to learn how to avoid this im tired of listening to the same songs