Once you try a few of the different routes you mapped out, you'll get a feel for which one you like best. I commute in the city of SF and added about 1 mile to the basic route that I take home, and I now have a route that is "quiet" (for SF standards) most of the way.

As for left turns, I play this by ear. If there isn't much traffic, you can do a left turn with traffic. With lots of traffic, I find that people get impatient with the bike and will try to get past me. In those circumstances, I just stop at the corner, wait for the signal and generally walk my bike across the intersection in the crosswalk.

For supplies, definitely bring everything you need to change a flat, just like thinpaperwings said. Spare tube, pump or C02, tire levers if you need them, and handiwipes...

Are there any bike shops along your route? You might note their locations in your head just in case you ever had an emergency.

best wishes! I always feel great after my ride into work.