not starting, occasional engine off

The car always off immediately after starting. It sounds as if it has started, but the rev will drop to zero immediately, except I press the accelerator pedal. Even at that it will still drop, until I depress the brake and engage the gear. Sometime the engine do stop while driving. Also It consumes fuel, the claimed consumption for highway is 6.6 ltr/100km. But now it does around 12ltr/100km.

Note: I bought this car used about 2 months ago.

Please, I am eagerly expecgting your response and advice. Thanks.