Re: First time firing up the engine, problem! :(
the plastic piece on the carb body is a secondary petcock. the fuel is turned on when it's turned up or down, and it's off when it's sideways.
your throttle cable is probably too tight or it's binding, that's why it's revving up.
check for binds or kinks, first. the throttle should turn smooth and free, without any "catching" or roughness.
since you know it's starting, check the little spring-loaded screw on the side of the carb. that's your "idle screw." turning it in increases idle (makes it rev higher,) out decreases it.
it should be set 3.5 to 4 full turns out. to check this, with the engine off, screw it all the way in till it stops. don't force it in, just till you can feel it stop. then back it out 3.5 full turns.
now, with your air filter box off, look into the carb at the slide. that's the little grey cylinder that's at the end of the throttle cable inside the carb. that moves up and down when you twist the throttle, opening to let more gas in.
it should be all the way down, covering the opening of the carb. if there's a gap, your cable is too tight.
there's a "barrel adjuster" on the cap that screws into the top of the carb that holds the throttle cable housing, with a knurled, round nut on it. loosen the nut and screw the adjuster and watch the slide, screwing it in till the slide stops moving down. now, twist your throttle slowly, and adjust the cable up or down a bit till it feels right. you should have a tiny bit of play before the cable pulls the slide up. just a tiny bit, any more and it'll be sloppy and you won't get full throttle. too tight, and it revs up like crazy at idle. then screw the little finger nut down against the base to lock the adjuster in place.
when that's all set, look back into the carb's throat and watch the slide as you screw in the idle screw slowly. it should move up slightly. if it does, unscrew slowly so it slides down, and unscrew till it stops sliding down. then screw back in slowly till it just barely moves up.
next step, looking at the slide again, twist your throttle slowly and watch to see if the slide moves up until you can't see it anymore. this is WOT, or Wide Open Throttle.
once you've got all this done, start 'er up. if everything else is right, it should be pretty close to perfect. you'll probably have to turn the idle screw slightly in or out to find the best idle.
now put the air filter box back on and ride it.
also, there's a choke lever on the front of the carb. down is off, up is on. that's for cold starts. you want it on, or half on when you first start it cold, then turn it off (down) after a minute when it warms up. after awhile, you'll figure out the best starting method for your bike.
if this seems over-simplified, sorry. just trying to make it as easy as i can...