The first thing to know is that the water should be boiling rapidly before adding your pasta. Another tip is to stir it a lot. Because pasta is covered in starch, the first few minutes is when the stickiness dissipates so you need to keep the noodles moving.  Some people add olive oil to their pasta as it cooks. While that makes your shells less sticky, it will also make them so slippery that sauce will not stick to them either.