The primary part of SC2 is certainly 1v1. You can play 2v2/3v3/4v4/FFA but they're all a bit random and generally benefit from playing with friends rather than random match making. The game gets very distorted when you start adding more races in teams, plus the maps are a bit poop. It's fun but 1v1 is really what the game is about.
You don't rigidly have to build a certain way to win, but there are certainly more efficient ways of going up the tech path to a specific unit or army composition and if you don't do that you might be behind vs. someone who is playing very efficiently.
At the low skill levels none of that really matters, it's just about learning the basics, cranking out a bunch of units and attacking (if you're Protoss or Terran), for Zerg you can also sit back and race to the late game tech and only build units if your opponent is going to attack you, but doing this correctly is of course a skill in itself.
SC2 is a lot of fun, but it also requires practice/regular play to be decent at it. You will need some knowledge and a good chunk of self motivation to continue playing as it is always punishing putting in effort and losing to a superior opponent, or losing because you made a sloppy mistake or didn't scout a key piece of tech and lost to a cloaked unit etc.
As for the Terran whine that is still infecting this thread, you're starting to sound like a 9/11 conspiracy theorist, "TLPD rig the stats so that the game looks more balanced than it is", really?!
Yes, Zerg is generally favoured over Terran right now, big deal. Terran was favoured over Zerg for just about the entire history of SC2, it happens. The game isn't perfectly balanced and never will be. Any changes Terran needs is to some of their late game units and they really do not need much of a change, a little too much and Terran will be back to dominating everything again so Blizzard must tread carefully.
Not even most of the pro players are at a level where they can say "the game cannot be played any better than I just did and I still lost". Players like MarineKing, DRG, Stephano and Mvp are all pretty close to that, but even they make mistakes and lose games as a result. It's interesting to note that TvZ still plays pretty well with similarly skilled opponents, but can look a bit silly with a skill disparity (much the same as Terrans would stomp slightly less skilled Zerg players with very little risk).
that was the 3rd nuke I tried. I decided to sneak the ghost round and launch from within his half of the map. He scanned in fron of his tanks where my previous 2 ghosts had been but he couldn't find it 