The main thing is, start with a good simple but solid story, and plan out as much as you possibly can. Do shot lists, number your scenes, draw storyboards… basically, create your movie on paper before you shoot it. A fantastic and FREE tool that helps you do this is Celtx (https://www.celtx.com/in
Second, when you shoot, you can use the native camera app or you can use video apps with more advanced features (like higher frame rate, a “slate”, audio monitor, etc.) like FiLMiC Pro (http://www.filmicpro.com
Third, you need good lighting and good sound, the two things that make a huge difference in shooting any film, large or small. Pick a good portable lighting kit from here (Amazon.com: videography lighting: Electronics (amazon.com)) and get a solid microphone that works with your phone (Like this one: http://tascam.com/product
I won’t recommend any editing software, as that tends to be a personal choice for most filmmakers, but I’m still amazed at how much you can do with the iMovie app… I’ve even used the iPhone version to quickly edit something and it worked great!
Good luck with your future filming endeavors!