Starcraft is EASILY the best RTS ever made, and considering SC2 is coming out soon, it wouldnt be a bad idea to play the original and develop a feel for the game. Statements like starcraft is about "who gets more units faster" are very ignorant, because a game thats so simple cant engender a pro scene or be played in mass amounts 10 years after its release. On bnet you'll still find that of all blizzard games, starcraft has the most users most of the time.
You cant play on bnet without a legit cd key, so either purchase one (SC is cheap now) or get hold of one from a friend who doesnt play it anymore. If you cant do that, visit sc.iccup.com (or iccup.com) and install their gateway. You can play there without a cd key. Alternatively you can also play on hamachi. (Iccup has LAN latency, similar to gg-client or hamachi).
I've played a lot of the CnC games, and while they're fun to casually play singler player, the multiplayer experience blows. I've played warcraft3 as well, and just the fact that a UMS (dota) is far more popular than melee clearly states it has much to be desired. One top war3 pro (non korean) stated himself that starcraft is a much superior game than war3 (google this, you should be able to find the interview).
If you feel like trying out SC, you absolutely must. The game is supremely intense, very diverese, has a massive skill range, and very fun. Once you begin to understand the nuances, you'll realize the true depth of the game and understand why its lasted so long. Plus, any skills that you gain in SC will 100% help you in SC2. I've seen some FPVODs of sc2 and its obvious that people familiar with SC will have a big edge.
If you're interested in reading a bit about it, or want to know how big SC actually is, try visiting teamliquid.net and gosugamers.net. Youtube also has a ton of SC videos, you could check them out as well.