Could you give a link to the pattern?
Are trying to convert from a flat sweater with a button opening or from a pattern that is seamed and has only an opening for the neck and arms?
If you just want to replace your straight needles with circulars ( never use straight needles) look at this video on YouTube:
http://video.search.yahoo.com/search/video?p=flat+knitting+with+circulars
Some articles on changing flat patterns to circular:
http://www.ehow.com/how_12116228_change-knit-pattern-straight-needles-circular.html
http://www.ehow.com/how_8644012_change-flat-knit-knit-round.html
With a link to the pattern it may be able to provide more information.