You'll be fine but it's a long, dry book. I'd start with "Engines of Creation", which is way more fun to read and provides all the basic notions. The Wikipedia entry mentions an "updated version" (from 2007), feely available online. The link is to a web archive of a pdf, but it works. I only recall reading the 1986 version, which I think is still very much relevant despite its age. A more recent introductory text by Drexler is Radical Abundance (2013), but I haven't read it. I say, read Engines, then skim through Nanosystems and keep it for reference, and get deeper into sections you find particularly interesting or where some frequent doubt or objection is addressed.