That's not a very specific question.
A not very specific answer would be: 'loving care'. Which you can apply to everything you should care about.
More seriously, since the required data analysis needs a supporting data model and the choice of the model may affect memory consumption and possible user experience performance, you should care to some extent. It's a big part of the job, not less then UI design and chart expression creation.
If you are asking about how you should start with data modelling, maybe have a look at
Get started with developing qlik datamodels
