There's no simple way to move already installed stuff to a second drive. I would advise against making the secondary drive your default installation path for all software. This is because if for any reason the drive fails or you somehow boot into windows without the secondary drive connected, you could end up with a bunch of errors and crashes because of the non existent paths. Stuff like steam games isn't a big problem as that won't break anything if it cannot be found.
Specifically for steam, you have to change the game installation folder from its settings. This guide from them explains it just fine:
https://support.steampowered.com/kb_article.php?ref=7418-YUBN-8129
As for most software out there, chances are that you might have an easier time just uninstalling and reinstalling on the new drive. Just make a folder inside it where you can put all software you want to install so you can easily distinguish between those programs and other stuff.
I would suggest that you only move the user's personal folders. Just go to your user's folder and right click on each one at a time. Under properties, there's a tab to select the location. It will let you chose a new path and will also ask you if you want to move current files or just use the new location from now on.
I could try to help with more specific situations but I guess this should cover most of what you expect to move.