Congrats on your new "baby," Chloe! You're going to LOVE it!
The lens should come with a little booklet that will give you some details, but mainly---you'll need to practice with knowing about how close you can get and still focus. In many, MANY cases, you'll need to manually focus when you get REALLY close or if you're photographing something with a lot of planes (depth of field) such as a lily flower. Flowers like daisies and such are more flatter, so auto focus will work fine in those cases.
The 105 is also an EXCELLENT lens for portraits, so don't limit yourself just to flowers and insects with it! It allows you to get head and shoulders (or closer!) without being right on the person and crowding their space!
On the left side of the lens (once you attach it to your D90) is a button that reads M/A and M. The M/A is for autofocusing. I leave it on that 75% of the time. The other is manual focusing only. It will be intuitive---you'll know when you need to use manual-only focusing. You'll be photographing something really closeup and the focus will go in and out and in and out because of a variety of reasons. The lens may not know what you want to be in focus.
Read up on depth-of-field and how to get the best results there. I put my camera on manual (on the camera, too) and do the aperture priority where I select the opening for the best depth of field. It's tricky, but since you have immediate feedback, you'll get the feel of it in no time. So play with aperture priority when you're doing the flowers and insect photos. That will help you make those beautiful out-of-focus backgrounds in your photos---the larger apertures will create that. But sometimes you'll want an complex layered flower (like a deep lily) as in focus as you can get it---that will also mean the background will be more in focus. Sometimes that's a plus, sometimes not. The biggest thing you'll need to learn is about depth-of-field when shooting really, REALLY closeup. Macro lens have more shallow depth of field.
So really, really, really learn about depth-of-field and how to use your camera in aperture priority or manual mode for this type of shooting. Most of the time you will use it in this fashion when you're shooting extreme closeups and bugs and such. In portrait mode, you can switch back to auto everything, if you like. But honestly----once I learned more about depth-of-field, I never went back to auto mode on the camera! (Yes, I still use both manual and autofocus on my LENSES.)
If you can, get either of the macro books I recommended on my blog a few days ago:
Practice, practice, practice. And please do send me your first efforts----I'm happy to help you out in any way---especially since I encouraged you to buy the lens! :-)
This was hard so many great shots. I Chose :HM John Daly's # 1 shot. I loved how the colors in the lines lead your eyes.3rd place Bobby Hamby's # 1 shot the River the water looks like glass and so sharp.2nd place Scott Fels Fence Line great DOf. First Place Rob Carnegie #1 shot the road. makes me want to see what is over the next…See More