Best shop is Deluxe at 1831 Market street. Good dudes work there and they cut the middleman so you can get any Deluxe brand board for $45. Just around the corner from there is Soma west skatepark, you'd recognize it, fairly new, super fun park. It gets crowded (not unbearably) in the evenings but mornings there are pretty empty. As for mission burritos, you can't go wrong with El Farolito and Pancho Villa, though there are many other really good options. I really like this place La Palma Mexicatessan for tacos, they have homemade tortillas that are fuckin fuego, and it's a short skate from Potrero skatepark where you can watch Jake Phelps drink a 40 and yell at kids.
You can hit China Banks, Federal Banks are fun, the library, Waller street ledges (a few blocks from FTC on Haight street), Pier 7 manual pads, the island if you wanna skate flatground in front of a bunch of tourists.. Spend some time in the tenderloin, there's really no place quite like it. You'll have the best time if you just skate around, there's so much to hit in between everything I've mentioned. The iPhone app Skate Savant has a pretty solid map of SF spots if you wanna check that out. Just avoid fisherman's wharf (unless you want in-n-out).
Have fun dude!!
Edit: Make sure you skate fort miley. It's so fucking fun