The title is not protected in most places, which means the qualification check is on you. It takes two minutes and it is the single most useful thing you can do before handing over money.
Qualifications and insurance
Look for a recognised certification — in the US, one accredited by the NCCA such as NASM, ACE, ACSM or NSCA; in the UK, a Level 3 qualification and registration with CIMSPA or the Register of Exercise Professionals.
Then ask two more things: current public liability insurance, and a current first aid and CPR certificate. A professional will have all three to hand. Hesitation is your answer.
If you have a medical condition, an injury or you are pregnant or postnatal, ask specifically what additional training they hold for it. General certification does not cover rehabilitation.
Session formats and cost
Trainers in major cities sit at the top of these ranges. Some also charge a gym access fee on top if you are not a member.
What a good first session looks like
A conversation before any exercise. Health history, injuries, medication, what you have tried before, what you actually want and how much time you can realistically give. Then movement screening, not a punishing workout designed to impress you.
If session one is a session that leaves you unable to walk downstairs for three days, that is a warning sign, not a badge of effort.
Questions to ask
That last one matters. Nutrition guidance beyond general healthy eating requires a dietitian or nutritionist qualification in most jurisdictions.
How long before you see results
Strength improves noticeably in four to six weeks. Visible body composition change generally takes twelve weeks or more with consistent training and diet. Anyone promising a transformation in thirty days is selling something.
Making it stick
Two sessions a week with a trainer plus one on your own is the format most people sustain. One session a week works if the trainer gives you homework and checks that you did it.