The lenses you buy should be driven by the pictures you're trying to take. When you're beginning, you may have a general idea of what you want to shoot, but the early days are for experimenting and figuring out what types of photos make your toes tingle. 

The recommendations for starting with what you have are spot on. Don't worry about more lenses until you run into a situation where what you have won't give you the image you're trying to get, then figure out what kind of lens will help you get that shot (by asking around this place, for instance). The answer will be a lot different if you're doing portraits than if you're doing street than if you're doing wildlife than if you're doing landscape than if you're doing macro. So don't make equipment purchases until you know what direction your work is headed. (invariably, someone always says "buy a 50mm!"; in fact, that's not necessarily the right answer. see here: http://photo.stackexchange.com/questions/14313/do-i-really-need-a-fast-50mm-lens/14315#14315 ) 

Also, before you buy, RENT. you can try out a lot of options by renting lenses from a place like borrowlenses.com or lensrentals.com, and see if that lens both solves your problem and is the right quality. 

But most importantly, focus on the image, not the gear. The gear is there to make the image possible (or easier). It's fun buying gear (we all know that), but unless you want boxes of stuff you rarely or never use, teach yourself to NOT BUY stuff until you have a specific need for it and you can explain how what you plan on buying will solve a problem between you and the images you're trying to take.