how to be creative in designing posters , visiting cards and ...

With no design experience, you should use Creative Cloud Express (formerly called Adobe Spark).  The learning curve is simple and you'll get great results with no design skills.

https://www.adobe.com/express/

Hi there,

Thanks for reaching out.
As you know creativity is something that will be uniquie from person to person. But as you have mentioned that you want to create something and you want some help, we will be more than happy to guide you. I would recommend you to complete some courses so that you know the tools properly, and once that is completed you can create you own content without hassel. Here's the link for LinkdIN Learning: https://bit.ly/3pZv2Ki

Let us know how it goes. 

Thanks,
Ranjisha 

You have taken course to learn the mechanics of the software, but I think your questions about how to approach the creative process could be best addressed by taking some courses in the principles of design, like this one. 

Hello @amir5E29 

Don't search about customers and you didn't learn how to deal with the software first. Learn, practice and make a good portfolio first.
- You need to keep learning.
- Search for inspirations (Behance | Pinterest).
- Make good portfolio. (The most important to find a job or a client)
Without making the steps above, don't ask about clients. The market is full of experts but there are more opportunities so think about what can you present to the client first.

From your name, I think you are Arabic speaker so this channel will help you reach the expert level in Photoshop and some basics of design:
https://www.youtube.com/c/OmarFathy_Graphics