1-there is many way to root the one x,the easiest is a one click tool
here,
and this
manual guide which is not completely for noobs (you need to know about adb and how to run commands)
but the best and the hardest way would be unlocking the bootloader via htc dev and going through the whole process manually,it's the best since other are based on exploits while this is official method and will work even after software updates,and here is an
how-to
2-if you used one of the two methods (not unlocking the bootloader) you will keep all your apps and contacts
if you unlocked the bootloader all data will be wiped but you shouldn't worry about contacts they are synced with your google account so when you sign in with your phone after rooting,your contacts will appear,just check that contact sync is enabled in settings and contact are synced with google (this is the easiest way but you can do a backup to the sdcard also,there is an option for that in contacts setting)
backing up apps without root is possible but you will not be able to keep their data,i think astro have an option to back up apps without root but with no data.
3-official roms is the last thing you need to worry about when you root,when htc release updates there will be RUU and stock cwm flashable ROMs and if you are planning to stay with the stock rom with root only,you maybe still get ota.