The speedlight's enough of a start, get it off camera!
If you're brave, cheap wireless triggers, if you want a safety net, use a long ettl cable. Then you need a light stand and a brolly and you're off.
Once you've mastered one light, and settled into it you can add another, triggering needs serious thought at this point, as does the limits of what you'll want to do. If you're staying inside a small studio flash with a soft box and wireless triggers, or you can stick with speedlights and get something that'll act as a master in a ettl setup. You can consider ettl triggers, or stay manual.