in general, yes.... all of your settings, contacts and data stored on the phone (not the sd card) will be lost. this is because when you install a completely new rom you should always do a phone wipe and cache wipe. not doing so can potentially cause issues with the new rom not working how it should.
yes rooting your phone can potentially void the warranty however you can unroot the phone as long as its not doa.
in your phone book press the menu key. you should be able to get to a section where you can save a file with your data. titanium backup can also pull all of this information.
it is always advisable to pull all of the data off of your phone and back it up just in case. look at the steps in my other post.
for your first rom i would suggest something well known and stable such as cyanogenmod or miui. once you are familiar with the process you can change roms in under 10 minutes.