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 10.10, Yosemite.
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.)
Install the Adobe Flash Plugin to enable your browsers to display Flash content. Set its control panel to update automatically: there are many malware attacks that try to trick Flash into infecting your computer.
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:
Hitting F12 brings up the Dashboard, showing "widgets" you have installed. I use Dictionary, Calculator, weather, phone lookups, and system status. You can get tides, movies, etc.
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.