Watch both 2001: A Space Odyssey and 2010: The Year We Make Contact.
Although 2010 is a sequel of 2001, they are completely different movies. 2001 requires a lot of mental discipline and is quite thought provoking. 2010 is a lot more linear and more-less spoon feeds you every step of the way.
I struggled with 2001. I even hated it after the first watch (I didn't get the point), but I watched it again, and with enough reflection, it's a pretty good movie (although, I still have a few questions, but I guess that was the director's idea). I wouldn't put it on my greatest movies list though.
2010 was not hard at all. I liked it, although it had its silly moments. I guess a 25 year-old Sci Fi is allowed a few mulligans.