How could I make sure that certain data are only for certain users available?
The answer is SECTION ACCESS. Section access is a connection between an authorization table (commonly placed in a hidden script and/or outsourced in an include-variable) and the application data (most to a dimension table).
Further you need to enable section access within the document properties in tab open and especially to enable the strict exclusion is important then without this is section access more a comfortable usability function to avoid confusing by the users which needs only a small data-area from the application but not more suitable for real confidential data.
Between desktop and server are differences which you should consider: Section Access: Desktop vs Server.
In addition to these there are more complex restrictions possible but before you build one think on alternatives like whereby those applications then have a specialized and simpler section access.
Restrictions based on section access could be applied on sheets and objects.
Sometimes there is a need to mask or de-identify data for certain users, for this see: Mask or de-identify data for certain users using Section Access.
At least the most important remark: before thinking on section access makes one or more BACKUP's from your applications then there aren't any go backs by mistakes! One exception (which don't nullified the suggestion of having Backup's) could you find here: Bypassing QlikView Section Access – Infinity Insight Blog.
There is some content-overlapping within the above used links but I think the complement to each other is useful and of course you will find many more interesting postings here within the qlik community to these topic - the notes here are a good starting point to go further.