Welcome from Ontario, Canada. For just starting out I recommend a 6X24 ruler. This size allows me to cut across 45 inch material that is folded with the selvages together, that is if you have a mat big enough. You didn't say which size mat you borrowed so at least get a ruler the will allow you to cut all the way across your mat. The less you have to fold your fabric to cut, is better for a beginner. I started cutting with just 2 layers to get good at holding the ruler securely, cutting straight across the fabric with being VERY careful that my fingertips were not close to the edge.