You must have a sound understanding of exercises, nutrition and the ways to motivate others. For this, you can easily find material on the internet or you can read books.
Realise that, you might have to encounter bad phases as your students can get injured during a exercise, so you must take first aid training sessions and register in a CPR. In order to obtain certification, it is a must that you take CPR classes.
You should check the requirements with a reputed organisation like the Aerobic and Fitness Association of America (AFAA), the American Council on Exercise (ACE) or the National Strength and Conditioning Council (NSCC). It is strongly advised that you try to earn your certification from one of these organisations as this play a key role in helping you attract clients.
In most of the states, trainers are required to have insurance keep their clients safe. Therefore, you should gain personal liability insurance.
You should ask your friends or family members to take benefit of your services and do not forget to ask them about referrals.
You should decide your hourly rate and contact local gyms, if you want to get hired.