See, the funny thing about post-install rituals is that they're different for every person. I'm going to give you my recommendations--I feel very strongly about them and firmly believe that they're the best way to use OS X--but take them with a grain of salt. I don't want to bias you too much.
First, in terms of tweaks: In Windows, you use the start menu to launch applications that aren't pinned to to your taskbar. Starting in Lion, Apple introduced a new way of launching applications called "Launchpad." Launchpad is very similar to Windows 8 in that it takes up the entire screen and is designed to replicate the interface you'd find on a phone.
I vastly prefer the pre-Launchpad method of starting applications: Drag your "Applications" folder into your dock. Right click, and choose "Display as: Folder" and "View content as: Grid". You can now launch applications by clicking the applications folder on your dock. Perhaps more importantly, you can now "install" new applications by dragging them right into this folder.
You can repeat this process with your Documents folder for easy access to your documents.
Newer versions of OS X also hide a folder that applications use called Library. The problem with this is that if you have any technological literacy at all, you'll almost certainly need to access this folder. Just as one of many examples, Minecraft loads custom textures from a location inside this folder.
To unhide it, go to "Applications" -> "Utilities" -> "Terminal". Copy and paste the following text, and then press enter.
Code:
chflags nohidden ~/Library
By default, new Finder windows (Finder = Windows Explorer) open to this weird view of everything on your computer. I like to change this to my home directory. You can change this by entering the following into the terminal:
Code:
defaults write com.apple.finder NewWindowTarget -string "PfLo"; defaults write com.apple.finder NewWindowTargetPath -string "file://${HOME}/"
In terms of programs, you'll definitely want something extract .7z and .rar files. I use
The Unarchiver for this. The first time I open it, I like to go into settings and uncheck the file associations for everything but .7z and .rar files. By default, it's a bit over-aggressive.
That's pretty much it. There's a couple of other tweaks that I like to make, but they're fairly nonessential.
(The one other
major thing that I do is delete a bunch of the default Apple applications. I never use Chess and I don't like having it's icon take up space when I go to launch something. However, I seem to be the only one who's really bothered by this, and starting with El Capitan, it actually requires temporarily disabling System Integrity Protection, so it's not really something I'd recommend to someone else unless it really bothers you)