Viewing one for the first time is a bit daunting
The most likely part of your toilet that wears out is the "Flapper Valve". It's a weak link in the system. It can cause water to trickle out continuously, waste the water and, hike your water bill or keep your well pump working overtime for no good reason.
Also, most people tend to put off the repair thinking that it's difficult or nasty, it's neither. The water in the tank you are looking at is the same as drinking water. Anything else is cold water slime or possibly grit. Neither of which will harm your hands in any way.
As with all projects, read this and other informative writings completely, compare points made in them and see if you feel comfortable attempting and completing this project.
I recommend the use of Nitrile Gloves, they last far longer than latex in my experience.
Think safety and always wear some form of eye protection at all times when pursuing this repair. I cannot imagine how anyone might get hurt but then again that's why it's called an "Accident" and not a "Plan".
I would also like to point out that the images are not entirely dedicated to the text - you could view them in any order but I did want you to see them - yeah I know, but I'm doing the best I can given the circumstances. If this offends then please, avert thine eyes 8-D
Step 1: Identifying Issues
also-I have larger hands than a typical seven year old and when the tank is drained the ball and lever are in the down position and hanging directly in the path of the flapper valve that I want to replace.
So then #1 difficulty in reaching the problem/hand clearance and #2 would rather not take any chances with moving an elderly valve (and thereby making it leak).
You are thinking "But, I don't want to do this underwater or with the water pouring into the tank" Neither do I. Look at the piece of clothes hanger wire I've bent into shape, it supports the rod and ball/float. In doing so it keeps the two items out of my way and stops the flow of water with out shutting the Stop Valve.
I bent the center down for two reasons. #1 I didn't want to put too much strain on the interior (Fill) valve of the toilet by "hyper extending" the rod upwards and #2) having a slight "V" keeps the rod and float in alignment with it's original position.
Some are soft plastic and can be gently pulled away from the tube they are attached to.
Others are made from hard plastic and require being snapped on or off. Be careful with these as you don't want an errant / broken piece of plastic moving around and ruining the seal you're trying to achieve or potentially clogging your toilet after collecting some Flotsam and Jetsam - if you follow the way I'm drifting.
Hint - try to match, fairly closely, the length of the chain connection between the lift or trip lever to the flapper valve before you install everything.
If you follow this little bit of smarts you won't have to shut off the elderly valve (that could leak when you open it again), remove the float or remove the arm that holds the float, both of which have or will become brittle with age.
Best of luck - remember- nearly everyone goes through this - you'll make out just fine.
Remember the 3 laws of Plumbing-
#1) - - it runs down hill
#2) Friday is Payday
#3) Don't bite your nails.