Having a good understanding of data modeling enables you to write better queries. Use the articles found on this page to familiarize yourself with these concepts such as database normalization.
I would start out by reading Keep is Simple: Easy To Understand Data Modeling Concepts, once you have become familiar with those concepts, I would highly recommend reading Database Normalization Explained in Simple English.
Once you are familiar with the concepts, then check out
Data Modeling Principles in Action, it will put some of the theory in to practice.
All the examples for this lesson are based on Microsoft SQL Server Management Studio and the sample databases AdventureWorks and WideWorldImporters. You can get started using these free tools using my guide Getting Started Using SQL Server