This really is a loaded question. I have to ask what you want to do with it, but how do you know as your still just new and dont know what you "can" do with it... So the better question is;
How serious are you.....
If your taking up photography I have to warn you that if your SERIOUS about it, its @#$%@#$ EXPENSIVE. So... If your SERIOUS and willing to spend some buckeroos your best bet will be Canons 18-200mm 3.5-5.6 lens. its upwards of around 800 bucks though, but gives you a TREMENDOUS zoom range. Its not that great of a lens however so the price is a bit high IMO.
If your not quite sure your serious, or just dont have that much money to spend I'd suggest getting Canon's 50mm F1.8 prime. It has ZERO zoom funtionality (thats what "Prime" means) but it has incredable depth of field and you'll make some amazing shots with it. Its also alot sharper than most zooms (almost all primes are sharper than zooms) and its able to take good sharp pictures in very low light. Its also the only "cheap" lens you'll be able to buy that you'll still use if you end up a professional photographer. Best of all, I think you can pick it up used for like 20 bucks.
What camera do you currently have? The size of the image sensor of your camera will affect the effective focal length of any lens you buy. For example a Canon 60D has a 1.6X image sensor, so a 100mm lens will effectively be a 160mm, but a Canon 5d has a 1x, so a 100mm is a 100mm.