Hello Andrei,
As a person who was born with perfect pitch, I taught myself how to tune a piano. It took me about five years (not every day for five years, of course) to learn how to manipulate the tuning hammer (wrench) with a fine-enough touch so as to tune a piano to my own satisfaction.
Rest assured, if you undertake tuning a piano, you WILL break a few strings now and then! Most commonly, a piano tuner breaks a string because he has inadvertently selected the wrong tuning pin, and then proceeds to over tighten the (wrong) string by accident, and .... pop!!!, a string is broken from over tightening it. One must learn to be very careful to select the correct tuning pin for a given piano string.
If you decide to acquire a tuning hammer (wrench), you should be aware there are numerous "cheap ones" out there that are not good for tuning a piano with any deal of accuracy. Why not? That's because cheap tuning hammers happen to "flex" when you attempt to tune a piano string. Expect to pay approximately $150 USD for a good new tuning hammer. (You can find cheap tuning hammers for about 1/5th that price, but you will eventually end up purchasing a good-quality tuning hammer. Spare yourself the time and trouble -- and acquire a good-quality one right from the start.)
If you aren't shown how to manipulate the tuning hammer (wrench) correctly -- and I see a LOT of poor tuning hammer technique in YouTube videos -- you will end up "bending" the tuning pin ... instead of twisting it correctly.
* * * * * * *
Regarding electronic tuning devices, some tuners might be okay for tuning guitars, but they are not sensitive enough to tune pianos with the accuracy required to do so! Many cheap hardware devices (such as a Korg or similar tuner that retails for $20USD) are accurate to only about +/- 1 cent (one hundredth of a semitone); the best PIANO tuning devices are accurate to within +/- 0.1 cents (one-thousandth of a semitone).
Even if you are able to tune a piano using a hardware or software electronic tuning meter, the individual pianos' irregularities require that you may need to deviate from absolute equal temperament perfection -- because the pianos are imperfect devices in themselves. You have to learn to "listen" to what the piano is telling you; otherwise you may (at best) wind up with a piano that a tuning device claims is in tune -- but which still does not "sound" in tune!
Enough of my rambling,
Cheers,
Joe