In my younger days I made a lot of money through my photography even though it was a secondary job (my photography ran alongside my teaching career).
 
I suppose it's the dream of most photographers to make a living from the "hobby" and it can be done. The main problem as I see it is that the dream is to make money from the images we shoot for pleasure and think we are good at - nature, landscape, weddings etc etc. The problem there is that it is a very crowded market and the door isn't opened automatically for you. Whether we like it or not, whatever type of photography we do there are far better photographers out there doing the same type as shots. Some make it through the back door like Patrick Litchfield (great photographer but being a distant member of the Royal family helps) Likewise Lord Snowdon. Some make it through pure grit and determination, some because of the sheer quality of their work.
 
You are entering a world where all of these people exist and you have to prove your work is better. Apart from cost ask yourself - Would a client rather have a Heather Angel image hanging on the wall or one of yours?
 
How realistically do you rate the chances of even your best landscape photo being used as a postcard, jigsaw, chocolate box cover when you consider the tens of thousands of other images submitted? It is a very tough playground.
 
I did all the things photographer do - weddings, portraits, pets, etc. and I made a few dollars, I was also a freelance photographer for Trials and Moto-X news - a national paper, covering weekly Moto-x meetings and 2 world championships. BUT the bulk of my money came from my decision to take photos of the things other photographers either thought beneath them or didn't pay huge amounts.
 
I did regular work for a local solicitor (Lawyer) taking shots of peoples scars for criminal injury cases. I photographed wedding cakes for a baker. I did sequence shots and submitted them to DIY magazines - not the glamorous, build your own house photos but mundane things like changing the washer in an electric kettle - I can't ever remember having a submission not being used. I told an editor I was surprised that my photos and brief article of changing a fuse had been used - what he told me should have been obvious. Many of the people reading their magazine were single mothers, widows, men with no DIY skills at all and the simple things were very relevant to them. I duplicated old photographs for people, submitted local interest photos to the local newspaper. I even photographed local gardeners "prize" vegetables that they were going to enter into a local show. I did school drama rehearsals, dance school concerts and a local sea angling club Paid me a few dollars each Sunday to take a photograph of the competition winner (no hardship as I was fishing the competition anyway - and often won).
 
They were all itty bitty jobs BUT they paid money. You have to remember that someone, somewhere always wants a photograph of something and if no one else is prepared to supply that photo then you do it. Little, one off jobs can lead to bigger, decent paying jobs. I once photographed a horse and trap for a guy, I just happened to catch the horse "Stepping out" (horse lovers will know what I mean). That led to another guy wanting a photo. After not to long a period I was attending local horse and trap rallies (even asked to judge and supply print of best turned out entry) - and the best thing of all - I had the market all to myself because no other photographers wanted to be bothered doing such a meaningless job. That also led to doing photos for a guy who hand painted gypsie's homes - full circle - the gypsies started to ask me to photograph their horses. From small acorns....
 
What I am trying to say is by all means have a go BUT if you want to make a living then be prepared to take photos that make you a weekly wage - photos where there is little or no competition, the things no other photographers in your area want to do. They may not be well paid jobs but lots of them can add up to a wage. Then if you sell a landscape or nature photo that is a bonus.
 
Simply ask yourself this... do you want to make a living at photography or just sell photographs?