The main destination for shared rooms and roommates - as for most other things, too - is Craiglist:

• Craigslist - room & shares San Francisco 
• Craigslist - room/share wanted San Francisco 
• Craigslist - rooms & shares in San Francisco & Bay Area 
• Note: Restrict your search by area, neighborhoods and min/max rent to help you find the right match

Other useful apps & sites:

• Crashpad: find roommates and rooms
• Lovely: simple and intuitive website and app to search for apartments, set alerts to be notified
• ApartmentList: huge search engine for apartments, better layout than Craigslist
• PadMapper: apartment hunt via map
• Roomidex: find roommates amongst your friend's network via Facebook
• Livewith.us: helps you optimize your Craigslist postings

More information:

• Curbed SF - On the Hunt for Roommates or an Apartment? Check Out these Craigslist Alternatives 
• Oh Happy Day - 5 Tips for Apartment Hunting in San Francisco + Our Story 
• Berkeley Parents Network - How to Find Housing in the Bay Area 

Image by 55Laney69 via Flickr

The weather in San Francisco is relatively mild and dry in comparison to the rest of the US. However, as the City is surrounded by the sea/water on three sides, it is windier, foggier and colder than further south in the Bay Area.

Current weather in San Francisco:


Click for weather forecast
Overall climate:

• The coolest month is January
• The wettest month is February
• The weather can be pretty nasty in summer, starting in June, when it often gets often foggy and cold (but nevertheless dry)
• It's beautiful in September and October, with September being the warmest month

More:

• Sperling's BestPlaces: Climate in San Francisco, California
• WeatherSpark: Historical Weather For The Last Twelve Months in San Francisco, California, USA

Average rent in the Bay Area:

• Shared apartment: ~1,000 USD
• Studio/1-bedroom apartments: ~1,600 USD
• >2-bedroom apartments/(town) houses: >2,500 USD

Costs of utilities:

• Often utilities come extra to the rent (e.g. water, gas, electricity, waste management, gardening)
• For a small house (2 bedroom place) the utilities cost might be around $180 -$250
• for an apartment you might look at as little as $70

Bay Area communities by descending price range & common desirability:

• Ridiculous: Atherton, Woodside, Los Altos Hills, Portola Valley
• Expensive: Palo Alto, Los Altos, Menlo Park
• Pretty expensive: Mountain View, Sunnyvale, Cupertino, Los Gatos
• Somewhat expensive: San Jose, Campbell, Redwood City, Foster City
• Not cheap: East Bay
• Developing: East Menlo Park, East Palo Alto (roughly the area east of US101)
  Note: The times of East Palo Alto being the city with the highest homicide rate in the US are over (1992), but it's still developing, check the details on city-data.com, as mentioned below.

How to find the right place on Craigslist?

• Almost all accommodations are posted on Craigslist, brokers & realtors seem rarely to have unique offers
• Main rule: reply instantly if you find a potential match, good offers are usually gone within hours
• Example: you are looking for a 2-bedroom apartment within a 20 minute commute from Palo Alto
• Select the two relevant areas in separate windows:
• Peninsula: http://sfbay.craigslist.org/pen/apa/
• South Bay: http://sfbay.craigslist.org/sby/apa/
• Select the applicable neighborhoods by clicking on '[+] all neighborhoods', to stick with the above example of around 20 drive from downtown Palo Alto:
• Peninsula: Atherthon, Belmont, Los Altos, Menlo Park, Mountain View, Palo Alto, Portola Valley, Redwood City, Redwood Shores, San Carlos, San Mateo, Woodside
• South Bay: Cupertino, Mountain View, Sunnyvale, Santa Clara
• Check the Bay Area map on SFGate to see where the Neighborhoods are located
• If you're unsure, check the distance to your workplace from your potential future home using Google Map
• Select '2+ BR' to look for objects with at least two bedrooms (living rooms are not counted)
• Enter your max. rent to restrict the selection and even min. rent to throw out fake offers

How to find a place in San Francisco?

• SF Startup Survival Guide: How to Find an Apartment in San Francisco
• The Bold Italic: How to Conquer Apartment Hunting
• Oh Happy Day: 5 Tips for Apartment Hunting in San Francisco
  

Hints:

• Get insights on average income, house prices, crime rates, etc. on city-data.com (example: Palo Alto)
• Always visit the the place before renting (at least for longer than just a few weeks), attractiveness of locations can vary drastically from street to street, Google Street View can you give you at least a first indication
• Helpful apps/websites to find housing:
• Lovely
• Padmapper
• Zillow 
• Trulia 
• Redfin
• Take the traffic into account, when planning your commute - San Francisco has the 3rd worst traffic in the US (applying similarly to the whole Bay Area, e.g. US101): USA Today