My fiction writing professor told us that the number one rule of writing a first draft is to just write it. Whether you outline beforehand is up to you, but focus on telling the story and don't worry about how it sounds (you will edit after you're done). It's hardest to go from nothing to something; it's much easier to edit once you have something written. If you get stuck on a detail like how to say something or which word to choose, fill something in temporarily and move on. I often use brackets when I can't figure something out, such as [name of city] or [description of room] and fill in the gaps later. It's very hard to get through a first draft if you're trying to edit as you go.
That said, sometimes there are details that you need to iron out before proceeding. I've taken this first-draft advice to the extreme - I once had a 7-year-old jump out a second floor window without getting hurt and travel 20 miles in a few hours. I've overlooked a lot of "technicalities" and figured I'd edit them later, only to realize that the foundation of the story was unrealistic, that there was no easy way out without changing the rest of the story. It's one thing to do this with short stories, but when you're writing a novel, you don't want to be on page 200 and realize that your story has no basis in reality.
To avoid this problem when I wrote my book, I assessed the importance of everything that was questionable. I would ask, if I had to change this part of the story, what else would be affected? If I could change that part of the story and still continue forward, it was a detail to deal with in the second draft, but if the foundation of the story depended on it, I would research whether or not it was realistic and figure out alternatives.
Here are two examples:
1. In my novel, a group of kids at a summer camp were doing things that wouldn't be allowed. I had to figure out how the characters got away with it during the first draft because the entire book centered around these activities.
2. The story originally took place the summer before senior year of high school, but the characters progressed, they acted more like it was the summer before junior year. I thought about what would change if I made them a year younger and realized that the differences wouldn't affect the plot, so this decision could wait until the second draft. But if it had been the other way around - if the characters were acting their age but I wanted to make them younger for the plot, then that would be something to decide earlier because the characters would have to change.
Finally, if you're working on a novel and decide to change something that has already happened, it's better to change it going forward and then go back when you're done. My mom gave me this advice when I changed the plot 60 pages in, and it really helped because it's much easier (e.i. less discouraging) to go back and fix things when you've completed a story than when you've just started.