Housing advice? I'm a young post-grad moving to S.F. and could use some help.

TLDR; Which of these best combine 1) fun and 2) convenience to commuting south.

I know everyone probably has their favorite neighborhoods in S.F. I'm taking a job close to south s.f. and was hoping to live near a Caltrain station or close to one. I was wondering if I could get some words of wisdom from those of you who have experience living in San Francisco.

• SOMA seems like a convenient area for the Caltrain, is this an expensive neighborhood?
• I called someone about an apt near the 16th st. Mission Bart station and the owner said it "was a rough part of town". Are there areas like this you would advise against?
• I've lived in a lot of apartments via craigslist in the past. Is there a different resource you recommend for finding apartments?
• Where is the BEST place a 22 year old can live near South S.F.?
• Anyone renting out a small 1br or studio? :)

I understand if you find these questions annoying. Thanks for anyone's help.