Information is always key through your interface and it can help your playstyle a lot. Having good name plates when you play a DoT class for example is a must. Try having all your really important information quite central on your screen near your character.
You can find multiple different situations of me playing at https://www.twitch.tv/bangerz/videos/all in which you can see my UI etc.
The most important add-ons that I have in which you could use too are:
WeakAuras - A MUST have addon in top raiding, really useful for tracking cooldowns, debuffs etc.
BigWigs - Really useful in raiding, seeing when an ability is coming and being able to plan around it. Use LittleWigs for Mythic Dungeons!
ElvUI - A full UI package which is really easy to set up and allows you to customise everything to way you like it.
Zpets - An addon which tracks how many pets I have and if they have been Empowered or not. Really nice addon.
/cast Demonic Circle
/cancelaura Demonic Circle
To use any Demonology Warlock WeakAuras, type /wa ingame and press import in the new tab and copy the below strings carefully into the box and hit import on the box that appears.
Whispers in the Dark trinket
I don't really have many WA's for Demonology as I have an addon called Zpets which tracks everything for me, so I highly recommend getting this add on. If you feel you need anymore WA's a really good site to check is https://wago.io/.