I had a nice, long answer typed out before I noticed this was for FireRed. This will be more challenging than anticipated.*
Step 1: The Crocodile Hunter
The only compatible parent in FireRed is Totodile (and his evolutions of course). Who, unfortunately, is super hard to get in FireRed! But, as a first step I will tell you how to aquire him:
You have trade with an Emerald game; you are give the choice of one of the three Johto starters after you fill the Hoenn dex.. Pick Totodile, of course. If you don't have an Emerald game, ask around and see if a friend does.
Step 3: How I Met Your Father
Hey, guess what? Totodile doesn't learn Ancient Power naturally! He has to inherit the move from someone else! Which means more breeding! But the good news is that finding the right Pokémon to pass the move down won't be hard. You know those two fossils from earlier in the game? Kabuto and Omanyte? They can pass down Ancient Power to Totodile. Here are the levels they learn it at:
Kabuto: Lv 55
Kabutops: Lv 65
Omanyte: Lv 49
Omastar: Lv 55
Make sure you have a male one of these. If you don't, breed with a Ditto until you do. This is important so that your eggs spit out Totodile and not Kabuto. While you're at it, make sure you have a female Totodile too.
Now breed your Ancient Power Kabuto/Omanyte with your Totodile until you get a male Totodile with Ancient Power. Shouldn't be too hard because of starter Pokémon's high chance to be male.
Step 3: Lovebirds… or… Lovelizards?
Now breed your male Ancient Power Totodile with your female Charmander. If your Charmander is not female, chuck it in the Day-Care with Ditto until you get a female one.
Step 4: UNLIMITED POWWWAAAAA
If everything went according to plan, and you didn't die from frustration like I would have, you should now be churning out Ancient Power Charmander! Cheers!