Hi and welcome to KH.
Can you link to your pattern or give it's name and where you found it? Maybe you could post just a few lines from the pattern.
I'm making an assumption here that you've got both legs on the needles, knitted in the round, and it's time to join them together and start knitting the body. In that case you would cut the yarn on one leg, knit across the front or back, which ever it is, of the first leg with the working yarn and then start knitting the other leg with the same yarn, leaving the tail of cut yarn hanging. HTH
You can't knit the left sock first. Ever.