great advice from others, but here's my 2cents. I got my machine 6 years ago and hoped to make enough to pay for my own fabric purchases in retirement. I didn't really quilt for others for about 6 months as I was on that steep learning curve. One thing that helped me a lot was a local machine quilters guild where others were so willing to lend advice and be supportive.
It's definitely a feast or famine business--because of that I was told by a Gammill rep that in our area, depending on earning enough to pay off a big loan on a machine is always going to be iffy. In my area (KC region) there are many long arm quilters. Plus there are more and more quilters investing in their own sit-down or mid-arm set-ups it seems. And the addition of special rulers and feet for domestic machines has also cut into the business some. I agree that having robotics (I don't) seems to help both in quicker turn-around and allows the quilter to charge a little less thus increasing their business.
How to get customers? Through your guild(s); working with quilt shops that don't have their own long arm (I do some sample quilts for a shop in return for referrals and some credit at the shop); giving a discount to customers that refer new customers; etc. Having a presence as a company on social media helps--but you need to take the time to keep it updated or it's a hindrance.
You don't saw whether you already have a long arm or where you live. Location is always telling on what you can make--so locations have enough quilters to keep long arm quilters busy, others don't. Location also impacts how much you can charge and make $$. I'd suggest you talk with some long arm quilters in your area, see if there is a machine quilters guild, etc. That will give you a good idea for what you might face in your area.