I’m working on my first sweater vest. The pattern is at http://knitty.com/ISSUEfall04/PATTpetrol.html
I’ve split the upper back & front and ready to shape armholes. it says to continue in pattern as set and CO sts at beginning of the next 2 rows…shouldn’t this be BO instead? Cause the numbers for the stitches goes down instead of up.
Then it says to dec 1 st at every RS row, How do I know which row is RS and which is WS??
CO and BO are the same thing. CO=Cast Off, BO=Bind off. So yep, you BO those stitches.
I’ve noticed that in Canadian/UK patterns, they use CO and in the US, it’s BO.
The RS is usually the side your first row was done on…I’ve found that most patterns will tell you which side is the RS or WS.
As a new knitter myself, I usually mark the RS with a small safety pin or a little piece of different coloured yarn so I can tell which side is which.
But what’s helped me the most was learning how each stitch looked after knitting it.
What a purl stitch looks like, what a knit stitch looks like, what a garter stitch looks like, etc.
I can always tell my RS from my WS now because of that. 
After looking at your pattern, it does tell you which side is which. Here, it says,
"Main Body Pattern (worked in the round over a multiple of 7 sts)
Rnd 1: K all sts.
Rnd 2: [K5, p2] to end.
NOTE: When this pattern is worked back and forth after dividing for the armholes, Round 1 will become:
Row 1 [[B]WS[/B]]: P all sts.
2x2 Rib (worked in the round over a multiple of 4 sts):
Rnd 1 [[B]RS[/B]]: [K2, p2] to end.
Every row is the same."
I’ve bolded where it tells you which side is which.
Okay I see it now. Thanks. About to get started on it again 