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How can I send some chars to the printer?

Printer as in, that thing that prints out onto paper?

Urm, well i think its a little more complicated than your making out to be. Its not like puting chars in video memory with vga.

Lets see, device drivers. Your printer will use a format. Something like postscript i think, im not that fluent in how printers work exactly.
You'll need drivers to support communication. i.e. it must be connected somehow right, usb, parallel etc.

This all needs a lot of your kernel to be complete like memory management, driver management.

If you could come up with some form of device driver management, and a standard printer spooler system, you could probably make all printers in your os sort of 'dynamic' so 'chars' are converted to postscript (if necessary), which are sent to the print spooler, which is directed to the printer driver, which is sent to the printer.

Dangamoose.

Actually... it really _is_ just as simple as putting characters into video memory... at least if you're only looking to output text.

Assuming you have a parallel port driver, you can just write characters to it.  Not unlike:
cat "This will be printed on my printer" > /dev/lp0

Each printer will have its own language for special characters, etc.  If it's an Epson, I used to have a copy of the Esc/P2 language that most epson's use (esentially, you just send an escape character (char)27, and then a code (0xB for bold, for example (that's not right... just an example )

If you're gettin' more advanced, graphics wise, then you'll have to write everything in the specific printer's language.  There is no standard printer language, although most support/emulate HP or Epson's languages (and most/all of the expensive printers will print postscript directly).

Cheers,
Jeff

but, where can i find the printers' languages?

You can find a few of the more popular languages here:
http://www.wotsit.org/search.asp?s=printer

Or if you mean, which printer supports which language... you'd have to ask the manufacturer   It should say in the manual as well.

I remember, back in the old days, all the printer manuals used to contain an appendix which described the language, and usually gave some BASIC source code for controlling it.... those days are long gone

Cheers,
Jeff