Good Afternoon, All:
The first big scene of any adventure, particularly a One Shot, is the Hook. This is the scene that draws the players into the scenario, and gives them a chance to decide whether they want to pursue the rest of the adventure or not. If you don't have a good Hook, they will move on, and you'll never have the chance to run the rest of the adventure.
Of course, at Cons, many Game Masters assume that you've already accepted the mission, but I prefer to RP it when I can. However, for those situations where we assume the characters have already accepted the mission, this is the first scene they play out in the mission itself, and represents your chance to settle them into your game and your gaming style.
It's my preference to use the Hook scene to introduce the plot to the characters. Here's where they learn specifically what the task is that lies before them. If the plot is to retrieve an item from the ruins of an ancient stronghold, then here's the scene where they enter the stronghold. If the plot involves rescuing the cheerleader from a supervillain, then here's where we present the players with the fact that the cheerleader is not at home, but instead snuck out and went to the big Homecoming game. If the plot requires the party to take a local noble to the subsector capital to appeal for Imperial aid, then here's where the patron offers the party a job. I'm sure you get the picture.
For most non-tactical scenarios, I tend to make the Hook the scene that leads to the party getting hired. I look at the plot I've created for the adventure (or generated), and figure out all the basic parts I'll need for it: the patron, the target, the opposition, the authority figure, etc. I don't have long to cover my bases, so my general rule of thumb here is to introduce these elements as quickly as possible. How many of these elements can I introduce by the end of the first scene, the Hook for the adventure? Depending on the plot, I may be able to do all of them, but I am okay if I only introduce two plot-based elements. The rest can come up in the second or third scene (but definitely before we get to the half-way mark).
One of those elements, almost always, is the Patron, the guy that hires the party to perform the task required by the plot. I can do this in a number of ways: The party may witness the Patron having a problem with the Opposition I've selected for the adventure, and feel compelled to intervene or at least help the guy out afterward. The Patron may not be having luck with an authority figure, and so turns to the nearest competent heroes, the PCs, for assistance. Maybe the Opposition picks on the player-characters, and the Patron approaches the party after the altercation to make his offer. However I do this, I want to end the Hook scene with the players actively in pursuit of the plot itself. This is where I insure their buy-in, and that's important if I want the adventure to be successful.
Also, some players (okay, many players) prefer to be given large neon signs from their Game Master in regards to what direction their next adventure lies. Your Hook scene should give them that insight, so there's no doubt as to where to go and what to do next. Even if the ultimate goal of the adventure is to get a clue for another adventure, you should leave the players with a strong idea of what they need to do next in order to get the clue they're looking for. You won't have to do that after this scene, if you lay the adventure out reasonably well, but in the beginning, if your players have no clue what to do, then you are wasting valuable game time watching them get frustrated. As we all know as players, that is simply not cool.
Of course, as a GM, we should only point the way. It's up to the players to decide whether they are going to take it or not. This early in the game, if they choose to take another course of action, you still have time to introduce a new adventure. The only exception I see to this is if you are running a specific scenario at a Convention, in which case, you can be a little more heavy-handed here, but it's always better if they choose to go along of their own free will.
In closing, I would strongly suggest that you look at the adventures you've enjoyed playing through in the past as a player, or your favorite books, movies or TV shows that approximate the plot of your adventure. Look at how those plots started in terms of involving the main characters, and borrow from there. You may have to change some names or make a few modifications, but there's nothing wrong with stealing good ideas if it helps improve the enjoyment of the adventure for your players and yourself. Remember, in the long run, it needs to be fun, for you and for them.