My answer is akin to all members' responses above

Haki basically means spirit, or ambition, or colour. It's different than chakra concept so hopefully you won't get confused.
Everyone has haki but it depends on that person to manifest it and use it to the max. To put it simply, one can develop haki due to these following circumstances;
- natural talent/self-develop since childhood
- intense trainings
- traumatic experiences/unusual strong wish on something requires power
Correct me if i'm wrong, nakama

I feel the need to explain at least to this extent before some people misunderstand how exactly haki works.