How to sync iPhone

Hi all, after many problems with my new iMac, (my fault for buying wrong hardware) I now have everything somewhere near running well. The only issue I have is that I have a 16gb iphone5 and my two children have 16gb iPads all of which are full of stuff.

I'd like to sync my iPhone and the pads with my new iMac and it's new iTunes library however when I connect to one of these devices and hit sync, iTunes asks do I wish to delete all music, photos , videos and documents so that it can be synced with this iTunes library?

I don't want to loose all my documents or my photos particularly on my iPhone, I can accept that on the iPads as all the kids have is games which can be reloaded!

How do I get past this problem with my iPhone?

Thanks in advance!

Lee.