What's the easiest way to grind on lower levels.

I'm new to wow in general and want to know how to grind and level up quick. I'm human btw.

Think RDF is the easiest way to level new character by level 60, after that questing in Outland and Northrend will be faster, because of fly mount.

Questing combined with RDF every once in a while.

For 58 onwards, I recommend doing Hellfire Peninsula quests, those quest rewards may last till level 68. Those quests also gets you Ramparts and Blood Furnace quests, both are good instances and offer solid loot & exp.

68-71, I'd do Borean Tundra and get to Amber Ledge and do quests there to lead you to Coldarra, and The Nexus quests. As well as Azoj'Nerub and Ahn'Kahet: Old Kingdom quests. If you do RDF or two at those levels.

Since Hellfire Peninsula (Thrallmar & Honor Hold areas) and Borean Tundra (Warsong Hold & Valiance Keep, Amber Ledge & Coldarra areas) are PvP hotspots, don't stay there if it becomes unbearable.

Depends on what realm you are playing.
Last time I checked RDF reward gave only x1 XP rates on ICC. Not sure if it got fixed by now.
In case it's still bugged it's not worth to do rdf whatsoever on ICC because questing has higher XP rates.