Use the loadtestdata command like this:
django-admin.py loadtestdata [options] app.Model:# [app.Model:# ...]
Its nearly self explanatory. Supply names of models, prefixed with their app name. After that, place a colon and tell the command how many objects you want to create. Here is an example of how to create three categories and twenty entries for you blogging app:
django-admin.py loadtestdata blog.Category:3 blog.Entry:20
Voila! You have ready to use testing data populated to your database. The model fields are filled with data by producing randomly generated values depending on the type of the field. E.g. text fields are filled with lorem ipsum dummies, date fields are populated with random dates from the last years etc.
There are a few command line options available. Mainly to control the behavior of related fields. If foreingkey or many to many fields should be populated with existing data or if the related models are also generated on the fly. Please have a look at the help page of the command for more information:
django-admin.py help loadtestdata