Nanoha chronologically is: Just Nanoha (Series and Movie), A's (Series and Movie), (Reflections movie albeit unreleased), then there's a time leap to adult Nanoha with StrikerS and Vivid.
Precure release order is: Futari wa Precure, Futari wa Precure Max Heart, Splash Star, Yes! Precure 5, Yes Precure 5 Gogo, Fresh Precure, Heartcatch Precure, Suite Precure, Smile Precure, Doki Doki Precure, Happiness Charge Precure and Go Princess Precure (and Mahou Tsukai Precure for next year). Order you watch them doesn't matter except the sequel ones Futari wa Precure Max Heart and Yes Precure 5 Gogo the only problem arises if you watch the Precure Allstars movies has they have all the precures that existed at the time. There are plenty of people who have made summaries of what to expect from each series for newbies, I generally recommend starting with the original or Smile but another option is just picking a precure you think is cute and watching her series.