First a quick warning, preview builds are not stable and will have a number of bugs. It is highly recommended not to run this program from your main PC in order to avoid any data lose and added hassle of retrieving needed info. It is also recommended to start with a freshly installed Windows 10 OS, in order to avoid having to reinstall and additional glitches.
To Sign up you need to use a Microsoft account. On the PC login with the Microsoft account not a local one. Go to the Insider page and sign in with the same Microsoft account, then follow prompts to register. https://insider.windows.com/
Next on your PC go to Settings > Updates & security > Advanced options > Get Insider builds click Get started. Follow the prompts. If the Get started button is grayed out check the feedback settings you have set on the machine. Go to Settings > Privacy > Feedback & diagnostics > Diagnostic and usage data change the setting from Basic to Full or Enhanced.