To get started;

TinkerCAD

- free, web-based, 'child-like'

Web-based CAD

OnShape

- free*, web, more traditional CAD

- can do everything with this

Most people use, professional traditional CAD

- Autodesk Fusion360 (free*)

- Solidworks

If you're serious about actually designing, jump right to fusion or Onshape (use Onshape if you want the freedom of web)

Fusion and solid works are Mac & Windows only

*free comes with limited access to features and storage (how many projects you can be designing at once)

Hope this helps. Feel free to ask anymlre questions. This is my profession of almost a decade