To find a narrow, not too competitive niche, make a lists and combine.  Like in the example below (sometime of the criteria can be on the same level and on different levels, that's okay):

lose weight is the broadest

lose weight for men
lose weight for women
lose weight for boys
lose weight for girls
lose weight for teenage boys
lose weight for teenage girls

Pick one of these, then narrow it more.

Let's say you picked lose weight for women

they love how they look except for thigh, or butt, or
they're in shape but want to be in better shape
never lifted a finger, never did anything (they'd die if they had to run to the corner)
they're somewhat fit

So now you pick women and one of the 4 above and combine with the next level

want exercise
don't want exercise
want exercise but not at the gym
want exercise but no exercise equipment
want to build muscle
don't want to build muscle
want cardio only
want cardio and stretching
want cardio, stretching, and weights
want stretching and weights
want cardio and weights

And now you combine what you got so far with one or more of these

want/have

to fit in the wedding dress
after childbirth
heart problems
diabetes
no health problem
high cholesterol
high blood pressure
busy
not busy
lazy
over 40
over 50
over 60
thunder thighs
big butt
big belly
big hips


Now, if you pick "lose weight for women over 40 who have diabetes, have a big butt, are busy and lazy", you've got a pretty narrow niche.  You need to get a product that addresses those problems, make sure that they spend money online to fix their problem, write copy to convince them you know what you're talking about when you recommend what you recommend, but it's a damn narrow niche.  You want another one, combine the 1st level with 2 or 3 of the next level.

Just make sure they're not mutually exclusive, like you promote exercise to those who want a pill or a potion.