Which lens to buy for Nikon d3100?

I just bought the Nikon D3100. I want a lens which would give a lot of blurriness in the background. I'm new to photography so don't have much idea about which lens to buy. I do know about the aperture settings.

The 50mm f/1.8 is inexpensive and will give you shallow DoF. It makes a great portrait lens on a crop sensor.

The lens won't make it blurry, its the settings you use. You have to focus on the foreground
also aperature is how wide the lens is open and the speed it takes to close it… F16 is wider and will close slower than f32 think of f as 1so really its like 1/16 seconds and 1/32 seconds

i get alittle confused sometimes so just google f stop, aperture, shutter speed