Renault Megane revs drop and stalls

when i start my car, sometimes it takes a few tries to start it, it doesnt matter whether its summer or winter, hot or cold. and when its in neutral the revs dont sta at a constant level, they drop really low and sometimes so low that the car just cuts out. i can be sat at traffic lights and it just stalls! i have to keep my foot on the accelorator to keep it from cutting out. the revs also drop when im driving and there have been a few occasions where im trying to get out of a junction quicly or get on to the roundabout and my cars revs just drop and i stop right in the middle of the road! verrrrry scary! and then the revs just pick up and i shoot off lol whats up with it? i use good petrol and good oil and i always check my oil levels and its fine. also, the heating doesnt work in my car. whats up with my car?!