Guys, keep in mind he/she is asking for an easy to learn RTS, not "What is the best RTS game ever?".
At any rate, normally I'd recommend Starcraft but when I think about it, it can get complicated, and since it's 11 years old, it is a little lacking (ex. you won't get tons of helpful tooltips, and unit AI is very dull, meaning you basically have to micro manage every unit).
Every RTS has complicated stuff for first timers, so I'd say Warcraft 3's your best bet - the campaigns start real nice and slow initially.
Seriously easy to learn RTS games can be found on the consoles though, since they can't cram as much nuances as they can in PC games.