If you need to know how to install addons (assuming your on pc) -To do a manual install:
Start by heading over to http://www.esoui.com . Look through what they have to offer and download any addons which might interest you. Just be sure it's up to date and compatible with the current game client (if it's marked 1.6.5 you'll probably be good but double check to be sure).
Now pretty much everything you'll download is going to be compressed into a .zip or .rar file so you'll want to make sure you have some way of opening them up. I personally use WinRaR but there's also WinZip or 7zip.
Once you've got a way of decompressing the files. Open C:\Documents\Elder Scrolls Online\live(liveeu for EU server)\AddOns. That's where your going to want to extract the files to (On the off chance there is no folder labeled AddOns in the \live folder then make one).
Once you've extracted the files to the AddOns folder, start your game up and log in. When you get to the character screen, on the left there'll be an option tab that says ADD-ONS . Click that and it'll open a menu where you should see a list of all your installed addons. From here you can select which ones you want to enable/disable and on which characters. You can also set it to run out of date addons *use at your own risk.
Of course if this all seems like too much work then you might want to look into a program called Minion (http://minion.mmoui.com). It'll will do the installing and updating for you. I don't use it though so I can't give you specifics on how it works.
Addons that might interest you:
Destinations
Lorebooks
MiniMap
QuestMap
SousChef
Skyshards
FTC
Notebook
All these really do is supplement or enhance the games rather bare UI. Nothing over the top they just make life a lot simpler.