Start with easier patterns/blocks. Like everything in life, we get better as we practice. Being off by 1/8 inch shows up more in some quilts than others. Someone has already mentioned pinning or nesting your seams, you can also try a light dab of washable glue to hold your pieces in place until you sew the seam. You may want to give paper piecing a try if you want accuracy - something like a log cabin block with many pieces comes out nice and accurate with paper piecing technique. Watch u-tube videos and read about accuracy in sewing and cutting, you'll pick up lots of tips that you can try out.