Yes the addon pack is very useful for new starters:
https://www.nostalrius.org/addonsIt is about a 50 Meg download but it contains most addons that where used in Vanilla wow. I have noticed however some are out of date or bugged. For example, if you are looking for a bag replacement addon, Bagnon does not work correctly but OneBag does.
Each addon has its own directory in the ZIP folder. First you will need to unzip the 1.12.1 Addons folder to somewhere on your computer. Then, in that folder, all the addons are listed in alphabetical order folders from A to Z, so they are easy to find.
Onebag is a good addon to start with, it replaces your bags into one big bag and has a lot of other cool features. So if you open up the "O" folder, you will see a folder called "OneBag". Copy that (select it and press Ctrl + C). Then go into the folder where you have WoW classic installed, open the "Interface" folder and click open the "Addons" folder and paste the folder of the addon (in this case, Onebag) into there. If these folders do not exist you will have to either create a new character first in WoW or make them yourself.
Now, when you run wow you should see a new button in the bottom corner called "Addons". Click that and you should see Onebag listed, make sure it is enabled, run the game and.....voila, you have your first working addon
Most addons contain buttons around the minimap to open settings to change features of the addons. if not, you might have to use commands in the chat interface. more details are usually found in a text file in the addon directory.
Some of my favorite add ons include:
Onebag - (Combines all your bags into one)
Bartender2 - (A replacement hotbar interface)
Extended Questlog - (Excellent addon for questing)
TipBuddy - (Lets you change the position of tooltips and allows you to modify what information is displayed)
Xpearl unit Frames - (replaces the blizzard unit frames with bigger clearer ones with more information)
I hope this helps you get started. If you get stuck PM on Twitter and I will help you
@ScroolooseUK