Depending on what you had before, in my experience, my D90 was really confusing at first, but a month or so with it and 3 manuals, and I am figuring it out. Buy a manual...the Nikonians D 90 manual looks to be a good one. I checked out "The D90 for Dummies" from the local library, and it helped tremendously. As you may imagine, I am not a techy person.
Lenses...they are all good! Disregerd comments like: The (fill in the blank) is not sharp wide open. Most lenses aren't, till you get into the megabuck big guns. But, most are extremely adequate for almost any normal use.
The 50mm 1.8 is a good lens,and light; 75mm equivalent on the DX D90, but not something I use a lot.
For great utility, as was mentioned, the 18-105 is a good, sharp lens, as is the red-headed stepchild, the 18-135. My absolute favorite "walking-around" lens is my old 28-105 D lens. I know, it is only a 42mm equivlalent on the DX D 90, but I don't use really wide all that much. I like it better than my 18-70, which is on my wife's camera. Any of the four will be an excellent lens, and won't break the bank.
My buddy, who sells his photos, has the older, compact Sigma 18-200 that he uses almost all the time on his Canon, instead of several of his other lenses, especially the Sigma 150-500. (Can't say as I blame him, there!)
Aperture? Don't worry about it! I shot my granddaughter's indoor tennis match with my D50 and the 28-105,(3.5-4.5) with good results, and, DUH! I forgot to raise the ISO from 200.
Soooo...pick a lens, any lens that appeals, attach it to the camera, attach your camera to your shoulder, and sally forth to commit photography!