Depending on how much you shoot and what you shoot you need a few lens, I shoot Canon but these same things apply to all SLR camera users.
You need a good wide angle, something that you can get more depth with, I have an 16-35 that is super sharp, I beleive it is an f2.8, great fast lens for an all purpose day of shooting! I also would suggest a fixed 85, usually much faster f1.8 is what I have and it gives great, super sharp, great DOF, and with people it gives a falt image, which is what studio shots need! After that it depends on what you want to shoot, I dont use much zoom so the highest i have is a 35-125, somwehere around there, but that lens gets used the least of all the lens that I have. I also suggest you go with the pro glass, much better, so much sharper, you will drop a goo bit on it, sometimes 2 or 3 times as much as the camera but it makes all the difference in the world! Also if you are so inclined I love a good macro lens, it gives you a super fast lens for not too much money, usually pick up a good fast lens for around $500, and they are almost always super fast!
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