Making photographs for clients is about a lot more than just making a nice picture.
You also need to understand the tools you use to do that, and how to make sure the products your clients want to buy are made properly.
Start by finding out the pixel dimensions your D3000 makes images at. The information is in your D3000 users manual. It is my understanding the complete users manual for the D3000 is on one of the discs that came with your camera.
In fact, your D3000 can be set up to make images at 3 different pixel dimensions: Large, Normal, and Small.
Now, when you want a print made you need to specify how many pixels-per-inch (PPI) the print will be made at. The pixel dimension and the PPI determine the size of the print.
Next you need to learn about aspect ratios. Aspect ratio defines the ratio of the long side of the photo to the short side of the photo, in other words the shape of the rectangle. A square has an aspect ratio of 1:1, all sides are the same length,
Your D3000 makes photos in the 3:2 aspect ratio and the long side is 1.5 times longer than the short side. Photo sizes in the 3:2 aspect ratio are 4x6, 6x9, 8x12, 10x15, 16x24, 20x30. In other words you don't have to crop to print those sizes.
16 x 20 is a 5:4 aspect ratio (the long side is 1.25 times longer than the short side) just as an 8x10 is, but the 16x20 would be printed with fewer PPI than an 8x10 to make the 16x20 bigger. You would need crop a native 3:2 aspect ratio D3000 photo the the 5:4 aspect ratio.
This chart maybe helpful. Notice the 11x14 is an oddball size