Is Mission District in San Francisco walkable? Can I walk to everything or do I need a car?

You definitely do not need a car if you live in the Mission! It has some of the best public transportation in the city, including two BART stations, tons of streetcars and bus lines. The Mission is also one of the most bike friendly neighborhoods in SF, with lots of flat, designated bike lanes (although you'll have to share them with tons of hipsters on fixies). There are a million shops and bars and some of the best restaurants in SF, so you won't have to walk very far to get anywhere, and places can stay open relatively late because of the Mission's reputation for nightlife. You can get great burritos at El Farolito and Taqueria Cancun at 3 AM, or just hit up one of the many bacon-wrapped hot dog carts out on the street. Parking on the main corridors is awful, but you can park up in Liberty Heights or Dolores Heights if you're willing to walk up a few steep hills. You can usually find parking somewhere after 6 PM when the meters turn off, and there's a pretty cheap parking garage between 21st and 22nd Streets, just off Valencia. Seriously, this is one of the cheapest parking garages I've ever encountered in SF, especially considering how good the location is. Still, having a car is pretty unnecessary in the Mission, and probably more of a hassle than anything else.

The whole town is extremely walkable. Its only 7 miles across . Much better to walk as parking is almost non exoistent and expensive if you F-up!

Depending on where you are and where you are going, the Mission is very walkable! There's also decent bus/BART service, too. The Mission isn't hilly, which helps with the walking action. Most people stick to the Valencia street corridor, but there's also stuff of interest on 24th Street. I wouldn't walk around too late at night, though as that's when there's the highest chance for problems with unsavory elements...