I’m like you and have thought of doing this - mainly alterations.
Many people are starting realise that clothes can look so much better if they are altered to fit their body shape.
I have been sewing clothes since I was young so I understand most construction techniques.
I was going to start by watching tons of You Tube videos, practice on my own clothes (ones that don’t matter if I screw up), practice on friends and families clothes (for free), before getting paying customers. I would time myself so I knew roughly how much each alteration would take.
Then I would do an online search for local business alterations and see their prices to know what my competition was. I would even consider paying them to do an alteration to see their prices and quality of work. I think most people would assess you on the quality of your work and price, rather than having relevant qualifications.
I would be happy to start small as the replies here show that people continue to use the same seamstress as long as they can, so word of mouth is most important.
I haven’t started yet as I’ve been divorcing and have moved houses. I’ve ended up with a chest freezer I didn’t want - so I’ve also thought about an add on service where I offer to freeze clothes if they’ve got a moths infestation. Freezing is meant to be a good option, and most people don’t have enough room for bulky items in their freezer. My freezer can even take a good sized wool rug!