Are you on the back or a front piece and is it a pullover or cardigan?
If on the back you dec at the beg and end of the row because you have 2 armholes. If it's a single front piece dec only on one edge.
"then on 1 foll alt rows "
A 'foll alt row' mean every other one, so after decreasing on the next 3 rows, skip a row and dec on the following one.
"and at the same time dec 1 st at neck edge as set on 1st (what does it mean as set on 1st?)"
As set would be dec in the same way you did before - if it's 1 st in from the edge, continue to dec 1 st in from the edge. At the same time means you dec on both the armhole edge and neck edge.
"dec st at armhole edge on next 3 rows, " dec at the edge on the next 3 rows whether they're purl or knit.
"and 1 foll 4th rows" then on the 4th row after that.