Not every dataset you want to use comes as a shapefile, or in a spatial format. Often the data would come as a table or a spreadsheet and you would need to link it with your existing spatial data for use in your analysis. This operation is known as a Table Join and this tutorial will cover how to carry out table joins in QGIS.
We will use a shapefile of census tracts for California and population data table from US Census Bureau to create a population map for california.
US Census Bureau has various spatial extracts from the MAF/TIGER database. You can query and download census tracts shapefile for California.
Americal FactFinder is a repository of all census data for the US. You can use Advanced Search and query for the Topic - Total Population and Geographies - All Census Tracts in California to create a custom CSV and download it.
Download Census Tracts for California
Download Total Population by Census Tracts CSV
For convenience, you can also download a copy of this data by clicking on the following links: (right-click and choose Save link as....)