Mushroom growing is a sometimes frustrating business, but a lot of people seem to think that it takes a lot more than it does. You can make things more complicated if you like (if you're the kind of person that needs to buy $500 worth of gym equipment before exercising, this might apply) but the basics are pretty easy until and unless you start wanting to make things more convenient, or more professional, or learn enough that you want to start tweaking the inputs to alter your outputs.
To sum up, here is everything you must know before beginning:
It takes patience - Growing mushrooms can be a lengthy process, taking anywhere from 2 to 6 months before you can harvest your mushrooms.
Temperature and Humidity Matters - Temperature and humidity are two of the most important factors when it comes to growing mushrooms. Different species require different levels of both, and the wrong levels can result in failure.
Different Strains Require Different Substrates - Different strains of mushrooms require different substrates, or growing mediums. These can range from wood chips, straw, and sawdust, to compost, grains, or even cardboard.
Mushrooms Need Light - Contrary to popular belief, mushrooms need light to grow. The right amount of light helps the mushrooms to produce spores and develop properly.
Start Small - Growing mushrooms is a learning curve and it's best to start small until you get the hang of it.
It Can Be Messy - Growing mushrooms can be a bit messy. Expect to deal with a fair amount of water and substrate.
If you can deal with the above, and are willing to grow into learning more as you go, then there's nothing to stop you from enjoying a fun (if sometimes frustrating) practice for more natural, healthier eating that can easily grow into a full fledged business.