There are several ways of getting profit out of 3D printing.
Here's the few one
Printing on demand - it is the most basic way of getting profit out of 3D printing. It is just simple like someone has the layout or the design, the file or the stl file will then send it to you then you print it out then send back to them with profit in return. With this idea you must list your service in some various websites like makexyz, and so on. In addition you could develop an independent brand with this.
Sell your models - This is the simplest one since you have the full control on what would be the outcome. You could sell your product in free markets or online. Remember design your own models marked for commercial use.
Printing during the design process - This is the most complex one 'cause people will come to you having just a concept alone, the challenge goes with your creativity in thinking process to produce the client's concept. However you could charge your client here for more as the design are becoming challenging. Tips, outsource and you'll be good here.
Educate - Basic one, you just need to educate others, like a you'll stand like a tutor.
There are more to consider. Just a reminder for last the last time, charge your client on what they can afford.
In starting with the use of 3D printing you just need to have a lot of equipment first like the computer, software for editing, and a print that capable of your needs. You can make business for tarpaulin.