I'm 25 years old and have no desire to have sex how can I fix this?

Psychotherapy. If you are 25 and have no interest in sex, chances are you are a female, and you could have so-called hypoactive sexual desire. Some mixed evidence indicates that low dose testosterone may help. Psychotherapy can also help. You should also be evaluated medically to make sure you don't have some medical condition causing your condition (e.g. Medications, hypothyroidism, other).
See your doctor. It could be many things causing lack of sexual desire including: stress, depression, past experiences, fatigue from daily life chores, hormonal imbalances (adrenals, thyroid, estrogen, testosterone...), anemia, anatomical problems....Talk to your primary care doctor about this to get it evaluated or refered to an appropriate specialist.