Step 1 - establish your need
What do you wish you could do that you can't with your current lenses?
Q: I want to shoot things that are further, I can't I only have an 18-55
A: Get a telephoto
Q: I do alot of urban shooting and would love something wider
A: Get a wide angle
Q: I'm shooting shows and like, my ISO sucks and makes things icky. I want a nicer image!!
A: Get an lens that opens up wide. ie a 2.8 zoom or 1.4/1.8 prime
Q: I want one lens that can do everything amazingly cuz my images sucks now!
A: Find a new hobby. Seriously, there isn't one magical lens for everything. Jack of all trades, master of none.
Image quality (generally speeking) goes like this: prime 50mm > zoom 24-70 at 50mm > 18-200 at 50mm
Q: I have the kit 18-55, I love the range, but want better quality
A: Look at the 17-55mm if you have a crop camera, the 24-70 is you have a full frame, the 17-40 as well.
Step 2 - establish your budget
the more you spend, the better you get (usually). 55-250 ($300) < 70-300 ($800) < 70-200 f/2.8 ($2000)
Lenses are the most critical component in getting a nice image. I'd prefer having a cheap body with kick ass lens, then a kick ass body with a crap lens
Only YOU know what YOU need. So give more info