There are a lot of adjustments you can make to the way your computer works. Sometimes this is wonderful.
If you overdo customization, it can eat up a lot of time and become a maintenance nightmare..
This lesson is oriented toward OSX 10.12, Sierra.
Select. You will see icons for preference topics. Click each one and you will see what you can change. Mostly the default values are fine. (When you finish with one topic, click the icon with the array of little squares to go back to the list of all topics.)
Because of the many security problems with Adobe Flash, I don't recommend installing it. If you must, be very careful about getting it ONLY from Adobe and keeping it up to date.
In Firefox, Chrome, and Safari, I make a folder in the bookmarks toolbar called "daily" (by using) and add my everyday favorites to it. Every morning, I click on the folder and select to open all the listed sites, each in its own tab.
Make sure "Open 'safe' files after downloading" is NOT checked in Safari preferences. This is important for security.
Safari, Chrome, and Firefox support plugins, which you can add to your browser to change how it does things. For Firefox, I have installed:
It is useful to make a few changes from the default in the Finder preferences. I choose "show all extensions" so that file names will show the full name of the file, e.g. "Mail.app" instead of "Mail". I also disable the trash warning, to suppress a dialog box every time I try to empty the trash. I select "Customize toolbar" and add the "Path" widget to the Finder toolbar: it is helpful when you have several windows open and forget which is which, e.g. is this on my computer or a remote computer on my home network.
Starting with Yosemite (10.10), the login wallpaper is a blurred version of the desktop picture. You can change the login wallpaper by replacing /Library/Caches/com.apple.desktop.admin.png with a 1920 x 1080 72dpi photo (or whatever your screen size is). Then do sudo chflags nouchg /Library/Caches/com.apple.desktop.admin.png to keep it from changing back. The picture will appear sort of grayed out.
I actually built a little script to generate such login wallpapers for my computers, specifying a logo and a message with my mail address and phone number, so that if my computer is lost it might find its way back to me.
If you use the photo album screen saver, you may want your photos to stay up longer than 5 seconds. If you are a programmer you can do this. This hack is OS version dependent and requires some sysadmin skill. You need to install the (free) XCode tools from Apple so that you can edit plist files.