Titanium ore is the most desirable ore to mine for in World of Warcraft today
. It is the highest level ore in the game and is a rare node spawn of saronite ore. This means that titanium ore spawns in the location of normal saronite ore nodes at times. Because titanium ore is the top ore in the game it is used for a variety of high level recipes and sells for more than all other ores on the auction house (this can vary at times of course). All of this makes titanium what miners hope to get while they are out farming for ore.
The basics to know for finding and mining titanium are that it spawns where you will normally find saronite ore. So very basically anywhere that you mine for saronite ore you can find titanium. Don't expect to find titanium all over the place though, it is a rare spawn and you'll collect very few compared to the number of saronite you'll find while running around a zone a few times.
There are several spots you can set out to and collect titanium: Icecrown, Sholazar Basin, Wintergrasp, and Storm Peaks. My personal favorite for mining titanium out of these four is Icecrown. I've had much success running (actually flying) around Icecrown collecting lots of titanium along with loads of saronite. This is such a good place to mine for titanium and since there are very few places you can mine it anyway it is often crowded with miners. I've found that even one other miner can really throw me off in mining a zone. If this seems to be the case you can try one of the other zones I've suggested or you can try to log on during a low server play time.