Importing LT SPICE schematic design into Altium Designer 18
Just draw it again in Altium, with the real parts and database that will generate your BOM. There is no way to import much of value and it wouldn't save you much, if any, time anyway. 

Then you will need to make sure you can find or create footprints for each part, and do the PCB layout in Altium. 

Finally, you can generate the Gerber (and NC drill) files from the layout, from within Altium.