If you're playing a main rogue, and you're the main damage dealer in the group, and your DM is running your group through a necropolis sort of place......then your DM is screwing you over if he isn't already giving you some means of contributing. Unless this is a short side-trek, since the Core Rules assume an average party of adventurers will only face a minor or moderate number of undead, thus giving Clerics a chance to shine sometimes and Rogues an occasional foil to their mighty sneak attacks.
Other people have suggested some means of helping yourself, but they are of course subject to whether or not the DM is going to let you use or acquire those necessary feats, spells, and/or magic items. Likely requiring serious investment in Use Magic Device, too. I don't know of anything else useful to contribute in that regard. Rogues should not be expected to deal with masses of undead very well. Actually, I suppose you could multiclass next level into Cleric and work towards getting some Divine feats and other stuff to use against undead.