Easy CAD software which allows making models for 3D printing is available at https://www.tinkercad.com . A beginner level tutorial for Tinkercad is here https://www.tinkercad.com/learn/overview/OQW3E48IYB2HI9X;collectionId=O2C1PXBIQ2KHCOD . Working through this tutorial should get you to a point where you can make some basic 3D models in software. You can search in the "Projects" tab to search for other people's projects and step by step tutorials on how to make a similar design for each Project.
Things to think about as you design a model include:
After making a design, the design gets prepped with settings for the specific printer. The libraries will do this for you. The default settings the libraries will use are:
Essentially, at the libraries, we want the 3D model, and then we will slice it for you in software and make the printer file.
YouTube has many tutorial videos about 3D printing projects, including software tutorials, and taking a project start to finish.