One of the major differences in raids from the rest of the game is how much time is spent in combat, it is not uncommon for a raid encounter to last 6-10 minutes; this is a totally different aspect of the game then many players encounter as bosses in dungeons often fall over in under a minute, and quest mobs rarely last your opener.
Understanding your class rotation and how to sustain DPS is a key difference in a raid and where many players struggle initially. The good news is there are plenty of guides written on how to improve at this.
I recommend two main sources of class guides for getting ready for raiding, I also recommend selecting the default raid builds for your first raid to get a feel for the class, then after that, you can start to branch out and try different talent combinations as you are more comfortable with the fights.
This include talent builds, rotational advice, your gem and enchantment priorities and the consumables you should use in the raid. This is all very good advice and will be needed at other points in our journey.