The Pokémon TCG handles several things different from the video games. Simple answers to your questions would be quite misleading. So first things first:
Type: The Pokémon TCG has fewer Types than the video games. TCG Types often contain two or even three of the video game Types. The TCG Psychic Type contains the video game Psychic, Ghost, and Poison Types. Most cards also only have a single Type even if in the video games that Pokémon has two Types.
Weakness/Resistance: Besides the obvious like Resistance causing a flat -20 to attack damage, another major difference is that attacks have no Type in the TCG. So when you go to apply Weakness and/or Resistance, you look at the Pokémon's own Type, not what Energy when to pay for the effect.
Poison does no damage: Poison is a what is called a "Special Condition" in the TCG, along with Burn, Confusion, Paralysis, and Sleep. There are other effects that attacks or Abilities can inflict on a Pokémon, but those five have special rules. Poison in the TCG places a damage counter on the Poisoned Pokémon between turns. Though doing damage involves placing damage counters at the end, an effect that places damage counters (like Poison) skips a lot of steps, one of those being the steps where Weakness and Resistance are applied. So Resistance does nothing to Poison.
Stack or Replace: Some Special Conditions stack with each other, but any time you inflict a the same Special Condition on a Pokémon, like Poisoning something which has already been Poisoned, the newer of the two replaces the older. So you cannot use Spit Poison multiple times to increase how many damage counters are placed on it from Poison between turns. Some Pokémon place multiple counters with Poison! So be careful not to replace a stronger Poison with a weaker one. If the card doesn't specify, then it just places one damage counter from Poison between turns.