The best advice in starting any activity is: fail early. Do the stupidest, corniest, most-cliched, novel ever and you're already ahead of 90% wannabewriters who never wrote anything.

a lot of this depends on the way you write and work best, but i've always thought preparation is one of the best things you can do when trying to write something like this. i've tried writing a novel a bunch of times and the main reason i usually stopped was because i hadn't prepared particularly well and got stumped and disinterested after a while. going into it with a reasonably complex idea of where you want the story to go and what you want to do with it will avoid a lot of these problems and help you keep a better pace. that said, i think you definitely don't want to over-prepare, as the story still should be loose enough for it to surprise you, but it's really helpful always generally having some idea of where you're going next in the story so there's never a point where you're just wholly stumped on something.
 
i could really make a long post on this topic but this is the biggest issue i've encountered.

one of my very best friends is a writer and his thing is mostly short stories. i guess it'd be better to start with that, get used to developing characters & compelling narratives from the fragments and make little stories out of them. A novel is kinda a huge thing and like most stuff, the more you write the better you get, so don't just go diving in the deep end