The simplest way to create missions is to let your 2IC do it all for you.
You have many other options of course - including defining the rules he uses, editing the missions after he has created them, or even creating your own. (See options for deploying ships for more information).
This page just explains how to let your 2IC do it all.
First, you have to call up your 2IC.
There are two alternate ways to do this. Both assume you are already in your Admiral's office.
From the main menu on the blackboard to the left hand side of your office, click on "Deploy". Then, on the deploy menu that comes up, click on "Form Fleets"
Click on the theater map on the wall of your office. A full screen map view will appear. On this map, click on the "Form Fleets using 2IC" button at the top right of the screen:
Either of these options will bring up the 2IC help screen for forming fleets. The screen has some help text, some buttons, and a picture of your 2IC. Shown below is an example for the US player, relying on Admiral Spruance as his (very cautious) strategist:
The help text indicates that the 2IC is ready to "draw up an operational plan, allocating ships to fleets and determining their missions". It also says that the plan is being prepared "in accordance with our very cautious strategy".
You can learn how changing strategy affects this plan by going to the changing the strategy for missions page.
For now, it is assumed that you are following the simplest of all options, and are not changing strategy.
To authorise your 2IC to draw up a plan, just click on the "Yes" button at the bottom right of the sreen.
The assignment of the most suitable ships to fleets and the calculation of their movement orders, all in accordance with the overridng strategy, is one of the more complex calculations the computer AI must do in SAS. There are many variables to consider when selecting appropriate enemy objectives, including the cruising range of your ships. So please be a little patient while the calculation is being performed. (You can speed things up considerably, when you are ready to learn this, by setting your own objective hexes. see Editing mission parameters for information).
As the calculation is running you can see the rate of progress in a progress bar at the bottom of the help screen. It shows the progress graphically and describes the mission currently being calculated:
As soon as the calculation is finished, the Mission List appears. This has a summary list of all missions in the left-hand pane. Here is a sample illustration:
Note:The summary list shows all missions created this turn as "NEW", to distinguish them from any that were created on a previous turn and that are still operative.
Click on any entry in the summary list to bring up details of the selected mission in the right-handpane. See using the mission list for more information.
The Mission List can be used for more than just reviewing. You can easily cancel all missions, or selected ones, or lock selected ones and cancel the remainder. See cancelling missions in the mission list for more information.
Close the Mission List when you are done, by clicking on the close button: 
at the top right of the List.
You will now see the theatre map, where you can review the missions in even more detail - and also edit them manually if you want to. See map view for more information.
Congratulations - you have just learned the simplest way to form fleets of ships and give them all necessary orders. Your 2IC has done all this for you, taking probably less than a minute. The missions will be the ones appropriate to your strategy, and the best available ships for the tasks will have been selected.
You can learn how to guide or override what your 2IC has done. But in the meantime, you can be confident that he has done a very solid job. You may find that you are happy to leave it to him for quite a while, while you get up to speed with other areas of your command - such as ship design and construction, or infrastructure spending, or aircraft construction and deployment. Life is always busy for the Supreme Naval Commander!