Most of the time, beadwork doesn't start with a knot - the tails are left loose to be woven in later. It makes things a lot easier if you have a good way to keep the beads on the thread at when starting a project. A lot of people use various kinds of clips, but I prefer to keep things simple by just using a bead.
Thread on a small bead (it won't be part of your pattern.) Stitch through it again, being very careful not to split your threads. Tighten it into place where you want your beadwork to start, usually leaving 3-4 inches of tail. When you're ready to finish your project just gently pull the temporary bead off the end of the thread and weave in your tails.