In order to mega evolve Garchomp you need the Garchompite but you must first upgrade your Mega Ring and look for this Mega Stone at the Victory Road (outside). It can only be found between 8pm and 9pm and the TM Rock Smash is needed.
If you want to know how to upgrade your Mega Ring, simply check the related questions below.
No, Mega Evolution can only be used during a battle.
No it does not have a primal evolution instead it has a mega evolution
Xerneas does not have a Mega Evolution.
Have it hold the Sceptilite, then press "Mega Evolution" during a battle to Mega Evolve it.
It's currently not known if Infernape is capable of mega evolving.
Aerodactyl doesn't have a traditional evolution. It does, however, have a Mega Evolution, which you can evolve it to by having it hold the Aerodactylite and selecting "Mega Evolution" during a battle.
Nowhere. As of now there is no such a thing as a Mega Salamence.
Gible.
The chances of pulling a Mega Evolution card from Pokemon packs with Megas vary depending on the specific set and rarity of the card. Generally, the odds are around 1 in 72 packs for a Mega Evolution card.
That's unknown. Arceus is already too strong as it is now, it doesn't need a Mega Evolution, and holding a Mega Stone would prevent the use of the Multitype plates anyway.
Venusaur is the final evolution of the line beginning with Bulbasaur. Venusaur cannot evolve into another form. It can, however, breed (at the Pokemon Daycare) and produce an egg which will give you access to an earlier evolution. Venusaur can Mega Evolve into Mega Venusaur using the Venusaurite. Mega evolution, unlike regular evolution, is temporary, only happening during battle.
No, because Mega Evolution wasn't introduced until Pokemon X and Y.