Are the letters and numbers going to be fairly large, like for a young child's quilt where you want to teach the alphabet? Or do you just want to do something more delicate like a signature and date?
If the first, I would use a stem stitch to outline block letters and then fill them in with filling stitches that give different textures to each letter (heringbone, checkerboard. raised chain, satin, spider web, etc). If the second, the stem stitch would be good for fine letters or you could outline and fill all in stem stitch, use couching, or a satin stitch (with or without outlining in a split stitch first). Confused? This site will give you some basic stitches.
http://www.craftown.com/instruction/embroidery.htm There are lots of sites with instructions for others.
Also, take a look at perle cotton instead of embroidery floss. It lays down quite nicely and (I think) looks much better. Just my opinion, for what it's worth (embroidery was my first 'craft' back in the days of Erica Wilson). I also think it's best to do "very visible" embroidery BEFORE quilting, so the back of it is not seen.