This is my first time using circular needles to knit a scarf that has stripes going lengthwise instead of the usual width-wise. The pattern says DO NOT JOIN and I don’t know exactly what that means. Also when I get to the end of the low, do I turn the needle around so the knit work is in my left hand as is usually the case when I knit with straight needles. Thanks!
Your two questions are sort of related, actually.
The short answer is: yes, you turn the needle around so the knitting’s in your left hand and you knit like you normally do.
The slightly longer answer is: Circular needles can be used for two types of knitting. One type (knitting in the round) you “join” the ends of the cast-on row so that you just have one big circle that you knit around and around and around; this is used for things like hats, gloves/mittens, sweaters, and other round pieces. The other type (knitting flat) you just move the needles like you do with straight needles, so that you’re knitting back and forth.
The “do not join” just is a way of indicating that you’re doing flat knitting (despite the circular needles).