You don't...
No. Riolu did not exist during Generation III, which was the generation Pokemon FireRed & LeafGreen versions were set in.
Riolu and other 4th generation Pokemon don't exist in previous generation Pokemon games this means it can't be obtained in firered or any game before that.
Catch a Riolu and evolve Riolu.
yes u can get riolu in Pokemon heart gold and soul silver you catch him or her in the safari zone and you can catch lucario in the safari zone two.
you can't- you have to catch pikachu and evolve him
No, not yet.
Floccessy ranch
You can catch Riolu in Challangers cave. You can't enter challangers cave till you have beaten the eliet four at least once.
You haven't made it far then. You can catch old pokemon. Yes, you can catch Riolu in the challenger's cave after you beat the elite 4. He's rare though.
Safari zone.
Yes it is possible.
Anywhere that you can catch Riolu. Shiny Pokemon can appear anywhere and anytime; with a 1/8192 chance of one appearing.