When I get a new computer it takes me some time to feel comfortable with it.
I usually have a lot of things to change, e.
g.
rename the hard drive, change the desktop wallpaper, change the Dock, customize folders and their view, and set a screensaver.
Just after doing that - this is really my computer! Here in detail what you can do:

1. Rename your hard drive - press CMD + Shift + C in Finder and you get to your hard drive.
   Mark the hard drive and rename as you do with any other file
2. Change your desktop background - go to your system preferences, click on Desktop & Wallpapers and then click on the Tab "Desktop".
   On the left side you can see a list with folders where you can find pictures you can use.
   But you can also use your own image - just click on any image you like with your right mouse and choose "set as desktop wallpaper"
3. Set the Dock where you want it - go to system preferences and choose Dock.
   You can choose three different settings: the size of your Dock, the magnification, and its position.
   
4. Label your folders - you can highlight a folder with a label.
   For that do a right click on your folder and choose the color you prefer under labels
5. Use your own Icons - if you don't have any just go to Google and add "Mac OS X Icons" and you will find great sets.
   Mark a folder you want to change the icon for, click CMD + I and you get into the information window of that folder.
   Mark the new icon you want to add and press CMD + C to copy it.
   Now mark on top left the icon the folder has so far and then press CMD + V to paste the icon into the folder.
   
6. Folder size - Especially when you have a large screen this is helpful.
   Go to your Finder and click CMD + J to open the preferences.
   Now use the slide for the symbol size to increase or decrease the size of your icons.
   
7. Customize the Finder Sidebar - When you have some folders you need to open again and again it is helpful to have it in the sidebar because this allows you to open it with just one click.
   Drag and drop any folder to any position you like and if you want to get rid of your folder just click on it and pull it out of the sidebar and it disappears again.
   

There are many more things you can do to customize your Mac, but this is a good start!