A group of your mates can set up studios or register yourselves as companies and present yourselves more professionally in order to sell your services too.
My advise would be to start close to home, find out if any friends or family need photography services and ask them to help promote your business to their friends and family.
Setup a social media page where you share your photos (with the subjects permission of course!). This will be a great way to showcase your talent and get new potential customers.
Most of all have fun with it, best of luck!
Be very selective of what you publish in your portfolio
Have an annual plan to the number of photographs that you will publish.
As you see it exceeding the plan, be more picky and restrict yourself to the best of the best
Always have your camera with you. Take everything and anything.
Find your genre of interest.
Use resources like gurushots, ESP, pexels etc to test your creativity and skills
Get advice from professionals
Just like any other profession, some foundational qualities remain the same: Best Customer Service, Quality of Service etc..
Narrow your selection to the genre that you are passionate about each of those can lead to different paths of business.
From personal experience, I'll add that knowing your limits as a photographer is key. When I was working towards photography as a side business, I was asked to do group events, when solo portraits were my strong suit. Suffice to say, the group photos did not turn out well. That's a customer who wouldn't recommend me in the future. It doesn't hurt to have a few photography contacts on hand that specialize in certain work--e.g. fashion photography, landscape, etc.--so you can reference each other if you're asked to do work in the others' specialty.