Guidelines and help navigating the BART, CalTrain & Muni and buses?

Greetings! I'll be visiting San Francisco for the Lesbians Who Tech Conference in the Castro District (mainly at the Castro Theater) next week. I've only been to SF once and drove that time. This time I'll be navigating via public transportation. I'll be staying in a few different locations and just would like the best advice on navigating from;

• San Bruno to the Castro District
• Castro District to Montgomery Street near the Financial District
• Montgomery Street to San Francisco Airport

Of course I can always go back to Google but I wanted to ask those who use the transportation the most and things I should look out for.

Also where are the best places to get groceries? Or best cheap spots for lunch in the Castro District?