Sadly there is no current "standard" width you can count on. I plan my quilts on a 40" usable strip -- and sadly we really like 42"+ if we can get it, specifically for those 10.5" measurements (4*10.5= 42). So if you have a full 42" of usable fabric it will be much easier. You would need (1) 15.5 inch row, (1) 9.5" row (or 10.5), and (2) 12.5" rows -- you can't get more than (3) 12.5" squares from a row.
If you have that 42" and can get (4) 10.5" across -- you are doing great. Otherwise, you need 3 across. In the leftover from the 15.5" x 10.5" rectangles, you can cut your extra 9.5x10.5 triangle. From the extra from the 12.5" where you only took one square from the strip, you can have the remaining 10.5 x 15.5 rectangle on it's side -- or if you have the length of yardage you could just cut two 15.5" rows, taking the 12.5 square out of that.
How are you cutting? Are you using a rotary cutter and rulers? If so, what rulers. It can really help a new quilter to have the correct dimensions but I can understand not having a lot of rulers. I would say that in addition to a long 6.5" x 24" ruler you should have at least one large square -- 12.5 or 10.5".