Is it worthwhile to download graphic design software to design poster templates?

I would take the path of least resistance and use a tool like https://www.canva.com/ to get started.

If you outgrow the capabilities of that service, then consider purchasing a more powerful app.

All the best,
-Shaun

Start with a market survey
Make visual choices to key design and marketing elements
Play around with variations
One of the most effective tools you can use for branding your business is through a logo, brochure, website, etc...
Try using Canva or Pic Monkey to create your own fancy marketing materials

Hi, I would do a short market sounding with comprehensive effort, eg. via https://www.producthunt.com and searching for your specific requirements. After having selected a short list, you can then see the reviews on www.g2.com - they have a quite comprehensive coverage of software tools. Selection criteria do very much depend on your willingness to employ your own labor into it and how individualized you want to promote your brand. Indeed (as someone in the previous answers already stated), canva.com provides you really very helpful templates, where you can gain an idea, how you want to do your branding.
Another option would be to entirely outtask this activity by searching for professionals on upwork.com or fiverr.com
If I can provide further information or you need a consultation, please let me know. I am happy, to help.

Photoadking looks OK at first sight - it's a freemium model - don't buy anything until you've checked it out. Canva is the market leader so check that out as well. In my experience the issue isn't the software so much as the design idea, so check out their templates. If you're not sure what you want you could always hire someone with good reviews on Fiverr for a few bucks and see what they come up with.

I agree with what my fellow experts have already said however i will add one more thing.
Whilst i like a few of the templates from some of the websites i find that they are generic and everyone uses them. That for me does not give me the flair, style and uniqueness that i want to convey.
For that reason I have been doing something different. I have already authored 2 books and what I did was got a graphic artist to design my own image that I have created. That way I own the copyright , have my own brand and have the option to tweek it accordingly.
When I do that i actually feel that I have accomplished something and that my brand has a stamp on the internet.
Since your business is going to be ongoing and will gain momentum why not get a software like Adobe Indesign and make yourself flexible to learning. I am trying my hand at that myself