I´m soldering a self-made H bridge with several 547 and 557 TO-92 through hole transistors and I don´t know how high from the PCB should I solder the transistor.
I found a tutorial just telling that they should sit just above the PCB but has no explanation why.
If it's a hobbyist project, just follow what you see on existing commercial hardware. You want it to sit far enough above the board so that the leads are not stressed, either because they're too short and everything is jammed together in assembly, or because they're too long and the part is moving from shock and vibration.
Ideally TO-92 transistors should be around 1/10 to 1/4 of an inch (2.5mm - 5mm) above the board. TO-220 style transistors should have the leads sitting on their shoulders (unless they're bent 90 degrees and the transistor mounted flat).
If you really want to do it up right, search out the correct IPC standard for PCB manufacture -- that'll go into exhaustive detail (and then you'll know more than me!)
Usually you want them 1/10" or more for easy soldering (and soldering). More height also helps to put some thermal resistance between the transistor and the soldering iron.
I once made an assembly house put a transistor 1/2" (about as high as it would go) because I needed thermal resistance between the transistor and the board. The board had some sensitive analog electronics and the heat from the transistor was causing tempco error's and a 10C rise on the board (when it was near the board). Giving some distance between the transistor and board allowed the heat to preferentially convect, than to travel into the board.
If your going to place the transistor high off the board and get the board assembled, it might be advantageous to also have the boardhouse put teflon tubing cut to the height that you want the transistor to be and put it on the middle pin. This prevents shorts in the case that some idiot decides to bend the transistor and forces the board house to think about how high the transistor should be.
