I'm looking for something cheap and good near subway/bart for 1 month
Air BnB is a great choice to find a cheap place to stay for a month in SF. couchsurfing.org is another if you're looking to go REALLY cheap.
There's a hostel in town called the Green Tortoise Hostel that I hear a lot of students stay at here in the city. I do know it's clean and has great owners.
As you may or may not know, airbnb.com and vrbo.com are good places to look to find a rental. In terms of where, depends on where you need to go, Glen Park Neighborhood area is close to BART, but kind of far to walk to any of the popular areas of SF. Mission District is another option, which is closer to central SF, however Mission area is kind of sketchy at night, depending on where you are.
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SF can get a little pricey, so as Chris has said give Airbnb a go.
Your best bet for a low cost 1 month sublet is craigslist or working your friend connections. I'd try AirBnB too for comparison.
Check out Airbnb.com