Easiest route is jump on Interstate 70 and head west to St. Louis through Indianapolis. From there you have to decide on the weather. Going south you'll hit hot temps, going north you'll hit the mountains....
I would take I-44 west out of St. Louis to Oklahoma City, then I-40 west to Flagstaff, I-17 south and pick up I-10 west in Phoenix.
Being from Riverside, you should be used to the heat and any traffic you might encounter....safe travels.....Dennis