Fire is not intrinsically a force - it is a state of combustion, usually exothermic oxidation of a material.
If it acts like a jetpack, then the combustion needs to combine air and fuel, which then must be directed - in your situation downwards.
Secondary Superpower 1 - Fuel
Like a rocket then - but you are using the air surrounding your skin as oxidiser, combine it with something combustible in your body, say oils, hydrogen or a combustible salt. Your first secondary superpower would be to manufacture this or retain it, or survive without it.
Also stopping the reaction is just as important as starting it - as many a rocket scientist may attest, once it starts going this is harder than it may sound.
Secondary Superpower 2 - Skin
Once oxidised and hot, the air needs to be directed downwards. Perhaps hairs on your skin, or new scales, could somehow be shaped to direct the reaction downwards and therefore push you upwards.
You can also use this to steer, your arms could be used to direct hot airflow into other directions.
Secondary Superpower 3 - Damage to organs
Combusting obviously creates heat and a lot of force - you need to protect your body and organs from damage. The skin is the first organ requiring protection, cell structures must not be able to fall apart when exposed to high temperatures and pressures.
Don't forget the other internal organs, eyes, ears, hair, nose, and genitals - these need protection too.
Secondary Superpower 4 - Sensory perception
Obviously when engulfed in an exothermic reaction it would be difficult to hear, see or feel. Lack of situational awareness is the primary cause of airplane crashes - and you would be no different. Orientation, balance, sense of location and direction all need to be more acute and you must perceive distance and other objects beyond your 'flame zone'.
Optional Secondary Superpower 5 - Clothes
Your superhero might need some clothes, but these will interfere with all the above. They may need to magically appear to maintain decorum.