If you activate Chandra Ablaze's first ability, you don't discard a card until the ability resolves. You may activate the ability even if your hand is empty. You choose a target as you activate the ability even if you have no red cards in hand at that time.

As the first ability resolves, nothing happens if your hand is empty. But if you have any cards in hand, you must discard one. If you discard a nonred card, Chandra doesn't deal any damage.

If the creature targeted by the first ability is an illegal target by the time it resolves, the entire ability is countered. You won't discard a card.

You may activate Chandra's second ability even if your hand is empty. As it resolves, a player whose hand is empty simply draws three cards.

You cast red instant cards and red sorcery cards from your graveyard as part of the resolution of Chandra Ablaze's third ability. You don't choose which ones to cast until you're actually doing so as the ability resolves. You cast only the ones you want to, and you may cast them in any order. Timing restrictions based on the card's type (if it's a sorcery) are ignored. Other restrictions are not (such as "Cast [this card] only during combat"). Each card you cast this way is put on the stack, then the ability finishes resolving. Those spells will then resolve as normal, one at a time, in the opposite order that they were put on the stack. They'll go back to the graveyard as they resolve.

If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can pay additional costs, such as kicker costs.

Planeswalkers are permanents. You can cast one at the time you could cast a sorcery. When your planeswalker spell resolves, it enters the battlefield under your control.

Planeswalkers are not creatures. Spells and abilities that affect creatures won’t affect them.

Planeswalkers have loyalty. A planeswalker enters the battlefield with a number of loyalty counters on it equal to the number printed in its lower right corner. Activating one of its abilities may cause it to gain or lose loyalty counters. Damage dealt to a planeswalker causes that many loyalty counters to be removed from it. If it has no loyalty counters on it, it’s put into its owner’s graveyard as a state-based action.

Planeswalkers each have a number of activated abilities called “loyalty abilities.” You can activate a loyalty ability of a planeswalker you control only at the time you could cast a sorcery and only if you haven’t activated one of that planeswalker’s loyalty abilities yet that turn.

The cost to activate a planeswalker’s loyalty ability is represented by a symbol with a number inside. Up-arrows contain positive numbers, such as “+1”; this means “Put one loyalty counter on this planeswalker.” Down-arrows contain negative numbers, such as “-7”; this means “Remove seven loyalty counters from this planeswalker.” A symbol with a “0” means “Put zero loyalty counters on this planeswalker.”

You can’t activate a planeswalker’s ability with a negative loyalty cost unless the planeswalker has at least that many loyalty counters on it.

Planeswalkers can’t attack (unless an effect turns the planeswalker into a creature). However, they can be attacked. Each of your attacking creatures can attack your opponent or a planeswalker that player controls. You say which as you declare attackers.

If your planeswalkers are being attacked, you can block the attackers as normal.

If a creature that’s attacking a planeswalker isn’t blocked, it’ll deal its combat damage to that planeswalker. Damage dealt to a planeswalker causes that many loyalty counters to be removed from it.

If a source you control would deal noncombat damage to an opponent, you may have that source deal that damage to a planeswalker that opponent controls instead. For example, although you can’t target a planeswalker with Shock, you can target your opponent with Shock, and then as Shock resolves, choose to have Shock deal its 2 damage to one of your opponent’s planeswalkers. (You can’t split up that damage between different players and/or planeswalkers.) If you have Shock deal its damage to a planeswalker, two loyalty counters are removed from it.

If a player controls two or more planeswalkers that share a planeswalker type, that player chooses one of them and the rest are put into their owners’ graveyards as a state-based action.