Welcome to the world of industrial control systems. TIA Portal, which is the Siemens PLC programming software, can be downloaded from the Siemens website using the following link: https://support.industry.siemens.com/cs/document/109752566/simatic-step-7-and-wincc-v15-trial-download-?dti=0&lc=en-WW
There are two parts to the software. Step 7 is the PLC programming portion, and WinCC is the HMI programming portion. Before TIA Portal was created, these were separate programs, but have since been more or less combined into one package.
After installing the software you'll be able to activate a trial license that is valid for 21 days. This will give you access to all of the Step 7 and WinCC features.
A free version of Step 7 was created years ago called Step 7 Lite, but it has been discontinued and is difficult to find. Downloading TIA Portal will be your best way to become familiar with the Siemens programming software.