The first triggered ability triggers both when Glint-Sleeve Siphoner enters the battlefield and whenever it attacks. You don't have to choose only one.

is the energy symbol. It represents one energy counter.

Energy counters are a kind of counter that a player may have. They're not associated with specific permanents. (Other kinds of counters that players may have include poison and experience.)

Keep careful track of how many energy counters each player has. You may do so by keeping a running count on paper, by using a die, or by any other clear and mutually agreeable method.

If an effect says you get one or more , you get that many energy counters. To pay one or more , you lose that many energy counters. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.

Energy counters aren't mana. They don't go away as steps, phases, and turns end, and effects that add mana "of any type" to your mana pool can't give you energy counters.

You can't pay more energy counters than you have.

Some triggered abilities state that you "may pay" a certain amount of . You can't pay that amount multiple times to multiply the effect. You simply choose whether or not to pay that amount of as the ability resolves, and no player may take actions to try to stop the ability's effect after you make your choice.