I have compiled a great collection of surfacing videos, tutorials, and presentations.
Start soaking: http://www.swtuts.com/?s=surfacing
But to answer your question more specifically. I do this style modeling often, here is what I do:
1. Create an assembly file. Drop the 3D scan into that. Start a new blank part file, drop that in too.
2. Try and match up the key areas that you can draw 2D planes, so you can sketch on those. Use splines. Use symmetry. Use tangent constraints across the planes of symmetry, to make sure the two halves have a smooth transition.
3. This is a skill that you learn by practice, so review the presentations and videos that I linked to, and then give it a try.
We welcome you to the dark side. I always hear about people saying that surfacing is a dark art and frankly, I don't try to correct them (I don't see the need).
The best advice I can give is keep trying and don't get discouraged. There is more than one way to accomplish something, some ways are better than others, but it is sometimes application specific. If something isn't lining up or working out the way you would like, don't be afraid to just start a new part and try again from a different approach. I have parts that started one way, just to scrap it, and restart a different way. With experience, things just come together easier.