I finished knitting a sweater, but I find difficulties concerning joining the parts together. I would really appreciate some help 
It says:
Join shoulder side and sleeve seam by backstitching, omitting rib, join rib by top sewing
My question is: what’s “omitting rib”? And what’s joining by top sewing?
Welcome to the forum!
What is the name of your pattern and designer. It’ll help to see the rib in the context of the rest of the sweater.
You’re meant to use a backstitch on all the seams except the rib. For the rib, the pattern wants a different join, possible mattress stitch or another join?
Thank you! 
I’m knitting a Wendy DK green standup collar,
That’s lovely, a beautiful knit.
I don’t see any reason to top stitch the rib unless you intend to turn it up so that the inside shows on the outside. A turned back brim on a hat or turned back cuffs on a sleeve.
You could use a backstitch all the way or a mattress stitch on the seams.
https://www.etsy.com/listing/468333996/ladies-knitting-pattern-pdf-download
Thank you so much 