Evolution of Combat
The toughest part about coming back to Runescape if you quit around 2012 or prior. A drastic change to the combat system. It split the community between those who loved it and those who hated it. It led to the creation of OSRS (Old-School Runescape). Many reading this guide may have even quit due to the update!
It plays a major role in the game today - modern bosses aren't possible without it. You may want to ease yourself into it, but you will want to learn it in time.
A good place to learn the new combat system is with The Junesong's Video Guides. It will provide you with everything you need to know to master this new combat system.