Photography is hard to take from hobby to trade. You need talent, dedication and practice. You need to be good technically as well as creatively. You need to be a good salesman and a good marketer.
Also, you need luck.
With all that said, it shouldn't stop you from trying to pull in some extra cash with your pictures. I'll break it down so you can make it happen.
There are three ways to go about it: stock, artwork sales and services.
Stock photography means splitting revenue with the agency that sells it. You upload your photos, they handle the sales. Three of the sites you might want to consider are dreamstime, shutterstock and 500px. Keep in mind each service has requirements to upload so be sure to read the fine print. You're also going for quantity sales since your profit will be low on each picture.
Artwork sales means that you are selling your photography on printed material. Framed prints, mugs, etc. This is the most freeing way to make money with your photos. You control what and when you shoot, but you could have to handle the prep work like printing and marketing. The best sites are 500px, etsy and redbubble.
Photography services is the most traditional way for photographers to get paid. It can include wedding photography and any assignment work that you might do for someone else. But, like any business, you have to put in the time, pay your dues, get the experience. Try to start as an assistant for a working photographer. Hone your skills and always be on the hunt for clients.
The best piece of advice I can offer is follow what you love. If you love taking pictures of dogs, sell pet portraits. If you love fashion photography, offer to shoot models for free while you get your name out. Whatever it is, put your spin on it and keep shooting.