How do I get volt tackle?

I think you have to put to pikachu in the daycare, then the pichu may have volt tackle when it's egg hatches. Something like that.

Only Pichu can learn Volt Tackle, and that's through breedind =[

However, I will tell you how to make this possible.

First of all, you need two Pikachus/Raichus; one male, and one female, obviously.

You also need a Light Ball. This can RARELY be found on wild Pikachu in Emerald, meaning you might as well migrate two Pikachu from Emerald onto Diamond, with one holding the Light Ball.
NOTE: The chance that a wild Pikachu that you catch (in the Safari Zone, in Emerald), will hold the Light Ball is as low as 5%, so BE PATIENT with this.

If you would like a male Pichu to know Volt Tackle, have the female Pikachu hold the Light Ball. If you would like a female Pichu to know Volt Tackle, have the male Pikachu hold the Light Ball. I highly suggest you get a male Pichu, so that through breeding using THIS Pichu/Pikachu/Raichu (as long as he knows Volt Tackle), the bred egg (Pichu/Pikachu, at least) will know this move, with or without the Light Ball being involved.

So anyways, put the two Pikachu/Raichu in the Day Care (in Solaceon Town), and wait for an egg. Then, hatch the egg. You should get a Pichu that knows Volt Tackle.

No Pokemon can learn this attack by leveling up, by the way.

Only pikichau can learn it.

.............. where can i get the light ball in pokemon diamond?

Well you get it in trophy garden 5% chance

U have to use a honchcrows ability night vision and it will tell you what item pokemon are holding only catch a pikachu when the honchcrows says it's holding a lightball then breed it and the pichu should have volt tackle

Also use pikachus ability lighting rod to catch wild arceus he has a 0.23615% chance of appearing in a wild fight

i got a light ball in the trophy garden in diamond

No, you can't get it in Pokemon diamond. Well, as far as I know. Anyway you could get shiny pikachu off an event, and then trade it onto HG and get a shiny pichu that has the light ball. That's what I did and it worked.

Actually according to Serebii.net, there is a 5% chance that a Pikachu will have a light ball in Diamond

I got my light ball on SoulSilver because my lvl 42 rhyhorn used thief on the trainers lvl 88 pikachu, so I can make my pikachu learn volt tackle too
Pikachu
Male
Brave
Moves:Thunderbolt, Quick Attack, Iron tail,and Finally volt tackle

Hey, it's guest # 421397 again. Just so you know you get the spiked eared pichu on an event in HG and SS and if you get it you can battle giovanii the team rocket leader

Well um I've been hearing you have to go to the trophy garden and find a pikachu that'll be holding a light ball then you um get a female and male pikachu and breed them but one of them have to be holding the light ball. When you get the egg and hatch it you should have a pichu that knows vold tackle.

You use a female pikachu holding a light ball and a male Cherrim... I got that straight from one of the books..