Choosing a Fabric
On your first pieces I recommend that you choose a fabric pattern that doesn't have to be matched. Matching patterns adds extra complexity that you don't need to deal with on covering your first pieces of furniture. With that said, if you choose a very plain fabric, any mistakes you make, or any wrinkles, will show up more clearly. So, a fabric with variated colors, or a muted pattern, or even a floral (with an all-over pattern that doesn't need to be matched) would be a good choice to help hide any imperfections of the finished cover.
If this is your first sofa that you will be recovering, choosing a good fabric will help you do a better job. It is easier to work with a medium to heavy weight upholstery fabric. Choose as heavy of a fabric as your sewing machine will sew. Since most do-it-yourselfers start out with a regular home sewing machine, this may be your biggest limiting factor. If you will be sewing cording in the seams (which means 4 layers of fabric in most sewing, and up to 6 layers at the seam joints), then you may need to use a fairly lightweight fabric. But if you won't be sewing cording in the seams, then you can use a thicker fabric. Before you make your final choise of fabric, you may want to purchase a 1/2 yard of whichever fabric(s) you are considering. Then do some experimental sewing on your sewing machine to see how thick of fabric your sewing machine can handle and whether or not your sewing maching can sew cording in your seams.
Lifestyle of users:
As you are determining the yardage amounts, be sure to allow an extra amount for making mistakes. Even professionals make mistakes, and as an amatuer you also will make mistakes. Depending upon your skill level you might add approx 25% to 50% or more additional fabric above any already generous amounts. As a beginner you are likely to make mistakes (which is normal). As your skill improves you will need a smaller allowance. Even professionals commonly add an allowance of 5% to 20% more fabric for mistakes, fabric flaws, etc.. It is far better to end up with extra fabric than it is to run short of fabric for a job. Sometimes the fabric store or wholesale supplier may sell out of your chosen fabric. That could be a disaster, or at least a challenge.
Now, to answer the question, "How many yards will your sofa take". Look at one of the upholstery yardage charts that you will find here.
Cutting & Sewing the Fabric
There are at least several ways to plan your cutting that involve anything from using planning your cuts to using the old cover for your pattern. Below are two different methods to cut your fabric. Use whichever method best matches with your skill sets and your temperament.