Ranged touch attacks are just like normal ranged attacks, except that they ignore armor and shield bonuses to AC. They use the "Touch AC" value on the target's stat block. Note that cover bonuses still apply to regular ranged touch attacks (I know there's a line in the combat section about incorporeal touch attacks; ignore it for now).
You add your BAB and your Dex modifier together (along with any miscellaneous attack bonuses, like having the Weapon Focus: Rays feat or luck bonuses from active spells or anything else), roll a d20, and add the bonuses to the roll's result. If the roll is equal to or greater than the target's Touch AC, you hit and the spell affects the target as the spell description notes. In the case of Disrupt Undead, it deals 1d6 damage.
It's unlikely that you will encounter any creatures with Spell Resistance at a level at which Disrupt Undead is useful as an attack; however, note that their Spell Resistance will apply if you do.
Example:
Your level 1 Wizard, Joe the Enchanter, encounters a bunch of skeletons. The noisy one (the one from Army of Darkness, screaming "Kill the mortals!"--I'm using the standard Human Skeleton stats from the Bestiary) has annoyed him enough that he wants to destroy it. Unfortunately, it's standing around a corner, just within sight (giving it a cover bonus to AC of +4), and its friends are standing in Joe's way, making a melee attack difficult.
So, he decides to use Disrupt Undead, checking to see if it is within range before attacking. It's twenty feet away, so that's no problem. Joe has a Dex score of 14, giving him a +2 to his attack roll (since he's a 1st-level wizard, he has a +0 BAB). The skeleton's Touch AC is 12 (10 + 2 Dex), and it gains a +4 Cover bonus to AC from being around the corner, bringing its Touch AC to 16. Joe will need a roll of 14 or better (14 + 2 = 16) to hit the annoyingly loud skeleton. He rolls a 15, making the attack a success (note: if he'd rolled a 20, he would have a critical threat, and would roll to confirm just like a normal attack)!
After seeing that his attack is a success, Joe's player asks the DM if the skeleton has Spell Resistance (it doesn't, by the way). Learning that it doesn't, Joe rolls 1d6 for damage. He rolls a 6, destroying the annoying creature altogether!
If you have any questions, let me know!
