Virtualbox is the best for beginners. You would preferably need a lot of disc space for the VMs because they can take up hundreds of gigabytes. Download Virtualbox from the website for your machine, and if you are a Windows user, you need to search up "Microsoft Redistributable Visual C++ 2015-2022" and find Microsoft documentation on it and download and install the version of it that's compatible for your system. Then install Virtualbox. Then Acquire an ISO of your choice for a modern OS like Windows 7, then click new (creates a new VM) and insert the ISO from the folder it's stored. If it's modern and known, it should automatically detect what it is, then go to the harware page and the more modern the OS, the more memory it needs. Never use more than half of your memory and more than half of your CPUs, I say leave the hard disk as-is then save. Then click start to start your VM (Can take up to a minute depending on hardware)