There are so many great cartoonists out there that it's hard to pick. But I'd have to say the ones who have most influenced my own style are Peter Arno, Charles Barsotti, Sidney Harris, Henry Martin, Charles Schulz, Bill Watterson and Jack Zielgler.
To be honest I'm up to my elbows in my own cartoons. The best advice I can give is to check out every cartoon book in the library, figure out what makes you laugh and go from there. Oh yeah, and draw draw draw!