Nice pictures!
When you shooting in RAW you're seeing exactly what the sensor takes.
As you said
It's allows you to edit some settings after the picture was taken such as the white balance, the exposure, etc...
I recommend you to get familiar with the "depth of field" if it's available on the D90. It should be.
Try all the settings, see what's the difference when you use a F/3.5, F/11, when you use a 1/4000 sec or a 2 sec pose.
After for taking pictures, I always do this way.
1. Compose your pictures, using the rule of thirds (google)
2. Adjust the settings
3. Be sure nothing will ruin your pictures (people, electric cable, (depending on what your shooting))
4. The most important, Shoot!
5. Let breath your pictures, see them, but do your selection only the next day.
And If you choose to shoot in RAW mode you will fore sure need an editing software. (Aperture, Photoshop, Gimp,)
I really like the Nikon software Capture NX2. But be sure to have a nice computer
Ho, And one of the best way to get a better eye is to submit your pictures so people can judge them
@ Redwatch Yes it's a 80-200 F/2.8 but a Tokina, a bit soft at 2.8 but great at F/4. It's also a Tamron 17-50 F/2.8 nothing to say on this one, just GREAT. I might change my Tokina for a Nikon in the future, not sure if I will keep the 80-200 or switch to the 70-200 VR