Well, good advice for the most part but assuming someone is going to ruin their life if they pick up a chainsaw is far fetched.
All of us had to start some where, cutting an already downed tree into firewood is a good start.
My advice, first don't try to cut a big tree to start with, something in the 8" to 10" range or smaller is a good starter tree. Only try to make the tree fall in a direction that it has room to go and don't try to make a leaning tree fall in the opposite direction of the lean. Make your notch cut and very cautiously and slowly make the back cut until the tree starts to lean in the direction you want it to go.
ALWAYS PLAN ON A ESCAPE ROUTE so you can get away from the falling tree, some trees kick back from the stump, very few but your escape route will keep you covered if this happens.
If your tree hangs up in the canopy, leave it unless you have someone with you who knows how to get it down.
Watch as many of the tree falling videos on youtube as you can.
You have to have a sharp chainsaw, a dull one will get you in trouble in a hurry.
Never take your eyes off the cut while you are making it, no day dreaming. I was day dreaming a couple of times while limbing a tree on the ground, not looking at what i was doing and knocking off limbs as fast as I could. Next thing I knew I had cut through my blue jeans on my thigh and slightly cut my leg, it was close and could have been much much worse.
The best advice I can give you is to round up someone with some chainsaw experience and ask then to show you the ropes and go with you on your first few missions. Make a bow for them and they will help you forever, I call these bows "thank-you bows" and have given out bunch of them over the years, not for chainsaw work but help in general.
I never touched a chainsaw until I was 35 or so, a couple years later I cleared about 5 miles of hunting club roads of fallen pines after an ice storm, it took me 6 months of my days off and after work. In one 100 yard stretch of road there were 75 downed trees across the road. These trees were all at least 12" diameter and up.
You can never learn how to use a chainsaw if you don't try, but remember to respect the chainsaw, it is always the master.