How can i build an hho gas converter?

I can tell you that they do, in fact, work. I've seen substantial gains with the generators I've fabricated and installed.

You will need a few basic things. An electrolyser cell...a bubbler....and a means of switching it on and off. More complicated systems utilize PWMs and EFIEs...but they aren't absolutley necessary.

The more efficient your design the better the gains you will see.

Do not waste your time on hydrogen fuel cells or otherwise. They fall under the fallacy of "free energy" and there are hundreds of sites where you can find good evidence debunking this myth. You are much better off spending your money on fuel efficient tires, spark plugs, or a more efficient car. Sorry but "been there, tried that"