I would recommend using the Data Load Editor almost exclusively. You can do everything in the load script, whereas some things can not be done in the data manager.
To associate tables you simply need to rename columns so they match. So to associate on the first column of these tables you would do:
C as A,
It's always a good idea to associate on only one field, so if you have a need to associate on more than one field first create a concatenated field with all the parts you need, e.g.
A & ':' & B as Key,
D & ':' & E as Key,
You will find lots of information about associating tables in Qlik on the Community and elsewhere. Note that the scripting is identical between QlikView and Qlik Sense - so some of the best information will be that which was written about QlikView.
Hope that helps.