More seriously, you have two general extremes with most groups I've experienced falling somewhere on the spectrum between the two: tightly scripted to free form. Scripted games have predetermined paths - sometimes just one, perhaps a set beginning and end with a few choices in the middle, or possibly a branching tree with different potential endings. As that 'tree' of potential endings gets more complex, more dependent on player choice, and less predetermined it gets closer and closer to free form. Eventually nothing is predetermined and you have free form.
I tend to plan NPC goals but not plot events or endings. It gives me a framework to build on and gives the players room for choice and action which affect the direction of future play.
Try both, see which works for you and your group. In general, a tightly scripted plot can (not a requirement) be planned for in extensive detail while player driven plots require a different planning focus from the GM. Which works best tends to depend on the personality and creativity of all players (including the GM).In the past, I've had two types, one that prepares everything and has an awesome story to tell, with lots of planned out options based on how he thinks we'll react to his scenarios, and one that improvised everything (everything). They all had a lot of experience, so I guess they knew what worked for them, but as a starting out DM should I plan a lot or a little?
Personally, I tend to fail at running tightly scripted plots. Mostly because they bore me. I want surprises, they keep me engaged. That's me though - I've played with GMs who want (almost) every major 'choice' planned in advance and can't react smoothly to a surprise. This is what most published adventures are - they have to be planned in advance. They're still often successful, sometimes wildly so.
Whatever you do, don't be afraid to fail. Not everything will be great at first. However, as long as you recognize problems and learn from them you'll continually improve. Good luck!