First off, I apologize for the large amount of questions in this.. So I'm learning to DM and I am wondering a few things regarding backgrounds, and the transition from the character's background to starting the game. First off, for the sample backgrounds provided in the PHB, are they meant to be fleshed out? A few of them like the "Acolyte" background seem very fill-in-the-blank and suggest questions to answer in your background. On the other hand, the "Charlatan" background feels very complete, like you could just use it as is without specifying anything really. Also, do you answer the example questions in the Background section on page 125 such as "what changed?", "why did you stop doing whatever your background describes and start adventuring?", "Where did you get the money to purchase your starting gear", etc. IN your background? For example ending the background written for the Acolyte "After a fire started by an evil sorcerer burned my place of worship to ash I set out to vanquish evil and bring honor to my god. While I left the charred premise searching for any survivors, I found a mysterious bag full of gems under some debris with strange markings."? Is their any example 5e backgrounds people have made somewhere? Additionally, in the section "Customizing a background" (also on page 125) are these instructions on editing a pre-existing sample background to better fit your character or the campaign setting, or making your own background from scratch? I guess the rules can be used for both, but im not sure. And now my other topic, how do the characters start off in the game? I know that there is multiple ways for this to be done, but I'd really appreciate some advice or examples. I assume it is decided by the DM what the characters should start out doing, and where, not the players? Basically the DM bridges the gap between the background and the present with things they make up? Or do the characters start out immediatly following their background? What are some good ways party members can meet up?
My advice is to just have the players pick backgrounds to start, and they can always be fleshed out as you play the game. Go ahead and get started playing though, as you will figure out your characters as you go. For some good backgrounds, there are lots of them here in the various pregen characters from WOTC:
The backgrounds are there primarily as character background/story help. Exactly how much detail gets put into it is between you and your players. As a DM, they're also giving you adventure hooks for their characters. The more friends, family, places, and events they flesh out, the less work you need to do to fully populate the world. Use anything they come up with to set stories in motion - but definitely don't use it to "punish" them for coming up with interesting stories, don't fridge their NPCs without a good reason.
As far as starting out, I recommend group collaboration before characters are even made. You can be as straightforward as telling them, "We're starting in the medium-sized city of Griffonford, fairly metropolitan, with both inhabitants and visitors from all walks of life. There's a commotion out in the edge of town - goblins are attacking! The call goes out for heroes to aid the overworked town guard, stop the goblins, investigate why they're attacking, and prevent it from happening again. Several heroes stand up and rush to the rescue. I need you to make one of those heroes."
Another way, if you want them to start off knowing each other, is to write each other into their backstories. Each player collaborates with the person on their left to have a positive relationship, even if it was just once. Then a neutral or slightly negative relationship with the person two seas down the left. Or switch it up. The sage looks upon the criminal favorably, as their organization helped provide some hard to find alchemical reagens, but initially mistrusts the soldier, as there are rumors circulating about secret events that took place in the last war.
Making characters individually and then spending the first session or two trying to meet up might be fun the first time or two, but depends on how well you can shine the spotlight around so it feels like everybody's getting the share of game time they want.
Before anyone makes characters, they need to be fully informed of what the adventure/campaign will be about. Tales set on the high seas will have vastly different characters in it then a wooded wilderness adventure. After the players are fully aware of the tone of the game to come, have them create characters. After characters are created, you can do improvisational exercises to get the characters bonded to each other at least a little bit.
I pretty much fully agree with the people above,
The biggest part of your answers will take place in what is commonly referred to as "Session Zero" this is where you and your players try (I use this due to scheduling conflicts which often plague groups) to sit down together and build their characters.
I tend to prefer telling the group straight up "You all know each. and have adventured together for some time, maybe even years" because the biggest killer of groups and slower of games for me seems to be when people's characters have no reason to like, trust or help each other. Like the old "Why are the Paladin and Thief in the same party?" question. Let the players figure that out before you get started.
As for how they move their character's from background to level 1 adventurer, that can vary from backstory to backstory. I've got two pre-made characters I haven't gotten a chance to use. One is a Jeweler and Merchant, also a sorcerer, and he still is a jeweler and merchant by the time he meets the party and hopefully afterwards, since his profession was traveling to cities and selling his wares anyways. How he joins the group may change that, but I won't know that til I get a chance to use him. The other is a Ranger who used to be a King's Spy, but was banished from the kingdom, perhaps branded a traitor or ousted by a new ruler who was cleaning house, and having no skills of peace became an adventurer, perhaps with goals of restoring his place in the kingdom.
I made a Barbarian Sage for a convention game I was DMing, who was a respected elder and shaman of his tribe, and left to catalogue and observe the culutures of other races and kingdoms, feeling that such knowledge would aid in establishing understanding and peace between his people and theirs. He brought along his sacred weapon, which was also a badge of office (I used the stats for a glaive, but said the blade was the jawbone of some unkown and ancient beast). I also made an Entertainer Fighter who was a mistress of Fencing, song, and insults, based off a historic figure and the "most interesting man in the world" commercials, and a Folk Hero Warlock who had been given into the service of a powerful Fey Lord, who then later sent him into the world to work his will on the mortal plane.
Bottom line? Write a cool story, or help your players come up with one. There will be fuzzy parts, but that's only a problem for people who want it to be a problem, and generally they'll fnd ways to work it out to their satisfaction with help from you or their companions to fill in blanks like "Did I ever run into Goblins or Orcs before? What happened?"
To which I've been told the answer must somehow involve a plucked chicken, three and a half spoons, and the Ballad of a Blind Gnome. 
Thank you guys, this really helps me out!