First you need to determine if this is a bearing wall. That means "is it holding up any part of the house above it?". The wall might be holding up the floor above or even be helping to support the roof, etc.. If you have ANY question regarding whether this is bearing or not then do not proceed until you get someone qualified to come INTO YOUR HOUSE and tell you. This is the only way to tell. We can't tell from here and shouldn't even safely venture a guess.
Assuming that you are 100% confident that this isn't a bearing wall then do this:
- Layout (draw) where you want your opening to be. It is best if one edge can start approx. 1.5" inside of a wall stud on one side. This will create less work. - Make small tenuous holes in the area to try and determing if there is plumbing, duct work or electrical wires, etc. in that area. If there is, either don't do the job or hire the appropriate tradesman to come and move the offender. - Cut away the plaster and laff or drywall along your layout line. - Remove one or two studs, depending on how wide your opening is. - If the wall is bearing then you need to remove all the drywall all the way to the ceiling (by the width of the opening) to install a bearing header, stud(s) and jacks - after temporarily supporting the structure above. - If the wall is not bearing then just remove the stud(s) and put a 2x4 flat header and bottom sill. - You might need to patch drywall, or if you were exceedingly neat (which is not common) OR, use wide trim to case out your opening (after you put a nice wide little counter/shelf on the bottom, probably with some brackets or robust metal L "let into" the bottom of your shelf).
I realize this is just a short description - comment back for details on any given aspect of the job. Your original post was kind of vague - feel free to use more words - we do...
See: http://popularmechanics.com/home_improvement/home_improvement/1999/3/window_replacement/print.phtml go down to "Widening the Opening" for some tips.