How can I isolate properly a home made PCB?
N.B: I'm still a 14 years old boy that is just a beginner in electrical engineering.

Excellent! Get a head-start by starting electronics at a young age :-) I did the same and never regretted it.

Indeed for high current you want the traces to be wide

For high voltage you want the distance between traces to be large

And don't forget that these traces can become hot alittle bit.

If that happens then you might need thicker traces. If that's not possible a way around that is to make the copper traces thicker using ordinary solder. That will only work if there's no soldermask on those traces. If there already is soldermask, you could carefully scrape it away (don't damage the copper!).

How wide should your tracks be for a certain current? It depends on the current, copper thickness and how much temperature rise you allow. Yeah, not easy, there's a calculator here.

How large must the spacing between the tracks be? There are calculators for that, here is an example.

For 220 V AC you generally need about 1 cm of distance between the mains connected side and the "safe to touch" side. Ideally there would be a slot (a hole in the PCB) in between.

What you could also do is look for similar PCBs, that can even be old PCBs from discarded equipment like a microwave, PC power supply or a TV. Look at those PCBs to get a feeling for how it is done.

Last tip, if you do not know the EEVBlog yet then you should have a look. There are several videos regarding circuit and PCB design.