Ambassador-get exalted with all 5 faction reps (orgrimmar, undercity etc for horde and stormwind ironforge for alliance) easiest way to get this one is to just farm the dungeons with the rep tabards on.
Explorer-explore every single zone that has an achievement, only really obtainable with the help of a 2 seated flying mount.
Diplomat-you need 85 help to farm the mobs for each of the three reps, this takes 20-30 hrs roughly.
Of the horde/alliance-need 100,000 honorable kills
The Bloodthirsty-need 250,000 honorable kills
The Noble-Do the noblegarden meta achivement world event
Guardian of Cenarius- Requires a LOT of mob farming for both rep and keys that are used for rep, generally 5 or so players interested in the title work together for this one. Also requires help to kill netherstorm elites. (this title is pretty much the level 19 version of 'The Insane'.
There are a handful of grandfathered titles too that you might see some 19s have, but I didnt bother listing them, the above titles are the only titles left that are still obtainable at 19 (i think i didnt miss any anyway)