Rename your Object something meaningful, as this will be the name assigned to your sculpt map.
In the top menu, select Render > Bake Second Life Sculpties. Hit OK on the "Sculptie Bake Options" menu. Your new sculpt map will show up in your UV/Image Editor.
In the UV/Image Editor menu, select Image > Save As. Save to your folder of choice.
Import the sculpt map into Second Life.
Baking
Before we get any further, you should rename your object to something meaningful. I've marked in yellow the Object name field on the image to the left. Click on the image to enlarge.
Your object is probably named something like "Cylinder." I've already renamed this mesh to "pawn." This is the name that Blender will assign to your sculpt map.
The Bake Script
Now go to the top menu. Select Render > Bake Sculpt Meshes (marked in red). You'll now be presented with the Primstar bake dialog. There are a lot of options, but the defaults should be fine, so just click the Bake button (marked in yellow).
You should now see a rainbow image in your UV/Image Editor division.
Now all you need to do is save the image. In the UV/Image Editor menu, go to Image > Save As. (Marked in green.)
Browse to your desired save folder and save the sculpt map.
Importing into Second Life (or other Grids)
Log into Second Life, or your grid of choice and go to File > upload image and select your sculpt. In the preview menu, select Preview as: sculpted prim, and it will show you a preview of your sculpt. If something looks wrong, check the troubleshooting section of this site.

These screenshots were actually taken in a grid called Avatar Hangout. Uploads there are free, which is convenient for tutorials.
Anyway. build a default box on the ground somewhere. Make sure that you're seeing the advanced tabs (click on More>>), then go to the Object tab (marked in green). Now change the Building Block Type to Sculpted (marked in yellow). Now you can replace the default sculpt mesh with the one you just uploaded.

This is my sculptie imported into an OpenSim grid. Notice it looks pretty much exactly as it did in Blender up close, but as I zoom away from it, it loses a lot of detail. This is referred to as LOD. With Blender, we can actually have almost complete control over how these different detail levels look. So lets go on to the next tutorial: Multires.