So I'm thinking about to get some old fluorescent lights from an old building that has been moldy for many years. Would that be any dangerous to the place where I live? The fixtures are obviously made of some kind of metal. And how should I clean them? Any detailed information would be good.

I think it's safe because mold doesn't live on metal.

I would not be that sure. If it is painted and with yearly layers of dust, the mold can pretty well live there.
But whether it is the case or not, it will be visible on the fixtures...

If you get them, take them straight to your yard (don't bring inside .lol. ) clean as much dirt/dust/etc off just using plain water from the hose.
Then if there is/was mold get a spray-bottle & put bleach in it (don't even need full strength, can mix it down 25% bleach/75% water) spray the fixtures with that, and let them sit for a few minutes. Bleach will kill mold. Then simply wash them with water again (may have to scrub some to get all the dirt/etc off)

Note if you do use bleach:
*use normal bleach, not the 'color safe' stuff.
*wear junk clothes just incase you splash any on yourself.
*don't do it in the lawn or a garden where plants are growing.

Thanks for detailed instructions. But can I take the fixtures inside after cleaning them? And I have read that bleach might be harmful to health.

Yes you can safely bring them inside & use them after they are clean & have dried fully.

Bleach is indeed harmful to health -- if you were to drink it or sit & inhale the fumes or stuff like that.

The bleach is rather volatile, so its fumes fill the room very quickly when e.g. it leaks or so. If you use it just normally, spending there just the time needed to do the bleach work and then letting it ventilate is OK even for indoors.

And if handling it only outside, you will be safe even when clumsy and spilling the whole bottle, so nothing to worry about. Also you won't get that much of the spores home.

You may want to disassemble it and look even in the inside and clean (the nonelectrical parts) the inside as well, depends how sealed or not sealed those fixtures are.

Also don't forhet the sunlight UV is quite a potent killer as well, so if you leave the parts that you can not wash exposed under the direct sun over the day, it will destroy the mold even there.