There are several good, free, online software options to help you build your timeline. The boxes below go over three relatively easy tools, but there are many more than what's listed on this page. Please use whatever tool is most convenient for you.
Canva is a free online design tool that lets you publish a variety of digital objects. There are several templates for timelines on Canva that can be adapted for your needs. I'm including a link that takes you directly to the list of timeline templates. You will need to log in or create a free account before you can access these options. Be aware that some templates require a Pro account, which is not free. These templates can act as a starting point for your content. Completed timelines can be downloaded as PDFs.
The video below includes a quick walkthrough of how to create a basic timeline infographic in Canva.
You can also use Microsoft PowerPoint to create a timeline. Microsoft has a straightforward walkthrough that describes how to use PowerPoint tools, specifically the SmartArt feature to complete these kinds of projects. Microsoft also has a variety of templates to get you started. PowerPoints can be saved as images or PDFs.
The videos below help explain how to use PowerPoint to create timelines in a variety of ways.
You can also use Google Docs to create timelines as well. There are a lot of suggestions for how to do this, but this blog includes a very clear walkthrough that describes how to create this type of project. This is probably the lowest-tech/easiest option for creating timelines. Google Docs can be downloaded as PDFs.
The video below includes a tutorial for how to make a timeline in Google Docs.