I haven't played for a long time but I can help you out in this.
Okay. Take a Pokémon card. Do you see it lists some attacks, and at their side there are some energy symbols? Well, these symbols represent the cost of that attack - it means it requires that amount of energies in that Pokémon to use it.
Now. Once during your turn you can play any Energy card from your hand, and attach it to one of your Pokémon. Each can have any number of energy cards attached to it. When a pokémon has as much energy as an attack says, it can use said attack.
For example. Let's say a Pokémon needs :fire::fire: to use an attack. It means it needs to have two :fire: energy cards attached to it. However if it needed :fire::fire::colorless:, it would mean the Pokémon would need two fire energies and one of any type: :colorless: means any type when dealing with energies. When a pokémon uses an attack, the energies aren't discarded though, they remain there.
Another thing is, retreat cost for a pokémon is paid discarding as many energies the card dictates. If a pokémon has a retreat cost of :colorless::colorless:, it needs to discard two of any energy cards to be able to retreat.
When a pokémon is KO'd, the energies go directly to the discard pile.
Hope it helps!
EDIT: Yes you can play energy cards onto benched Pokémon. You can play only one energy card in your turn though.