Typically I break down a job into discrete tasks (import data, add lookup values, filter, create output, check for errors).
Sometimes the task dictates the required component. Input and output components rely heavily on the format of the file or type of database in use.
Some tasks can be done in a number of ways, and which way you choose isn't so important as long as it gets the job done. You'll need to learn which components do what, but they're well documented (press F1 and click on a component to get a link to its documentation page) and often there are examples that show a simple use case.
It all depends on the use cases. Going through training will help clarify this a bit. There are also a lot of videos on youtube, www.talendforge.org, and many other sites.
In general:
Get data from source -> Transform -> Save data to target
