What lens to buy?

A frequent question from those starting out in photography. "What lens should I get? Which is the best lens? "

Read this thread from the Fred Miranda forums. Sums up the answer perfectly.

Basically, there is no best lens. The lens you choose depends on what you want to shoot and what you feel your current gear cannot provide you. As you explore various styles of photography, you will need specific lenses to get specific qualities in your images. Unfortunately, you can spend a lot of cash doing this. Beware!

For a lot of people, their SLR is really a big point and shoot with a nice sensor. Good for holiday pics and snaps of friends etc. A good zoom is ideal for these photographers, but while a zoom is flexible, there are many compromises. There is no lens that does everything.

Zooms have smaller appertures - means slower shutter speeds and reduced ability to create nice out of focus areas with shallow depth of field.
Zooms are not as sharp as primes, and have lower overall image quality (flare, chromatic abberation, distortion).
Zooms can't focus as close as primes - especially macro lenses.
Zooms can't tilt and shift.

So start with a zoom. Figure out if you can live with the compromises. If not, then find a lens which can do what you feel is missing in your zoom. When you start out, don't try to build a kit that will do everything. You'll probably get it wrong because you need experience to know what you need - everyone shoots differently and you need to get stuff that suits your style.

Enough rambling from me. Get out, shoot, have fun.