The basic To Do List module has many parts and each is documented in a separate page.
First, you will want to install the module. Note that you want to Rebuild the Post permissions. Once you are done with that step, the other sections are optional as the default settings make the system work as expected for the administrator user (i.e. user #1). If you want to allow others users to use your system, check out the Permissions and enable access to specific roles. You may want to create new roles for this purpose.
Sub-modules are available to add functionality to the basic To Do List module. These are documented elsewhere. Go Up once and select the module you want more information about.
Click on the image to see a 1:1 version.