Although many people think of Pokemon as a child's game, it actually involves a lot of strategy, forward thought, and even trickery to achieve a victory.
In the game of Pokemon, knowing types are essential to achieve victory. Each Pokemon is given at least one type, and each type is strong against some types weak against other types, or maybe even immune to some types. Knowing the weaknesses, resistances and immunities, is essential to playing Pokemon. There are eighteen different types a Pokemon can be, and if each type is strong against a few types, weak against a few types, or maybe even immune to some types, it's going to seem like a lot to remember.
How to remember the weaknesses, resistances, and immunities of all the types
Typically I play Pokemon on the website https://play.pokemonshowdown.com/, and on that site if you type "/weak Pikachu" for example, it will tell you the weaknesses, immunities, and resistances of Pikachu. However, if you're not playing on Pokemon Showdown it'll be a little more difficult to determine the advantages and disadvantages of a Pokemon's types, one of the things you can do is use a type chart, like this one:
If a pokemon has two types, it will take super effective and not very effective damage based on both of those types combined. For example, Sableye is a dark and ghost type, normally ghost would be weak to ghost, but dark resists so it takes neutral damage. Although Sableye's dark type is weak to fighting, it's ghost type makes it immune to it. If both of a Pokemon's types are weak to the same type, it doesn't take twice the damage from it, it takes four times the damage from the super effective type.
Other things Pokemon types do
In Pokemon, types don't just determine the weaknesses, resistances, and immunities of a Pokemon, they also give moves of the same type of the Pokemon using them a 1.5x multiplier this is called "Same type attack bonus", or "STAB". What this means is if a Sableye used the move "Psychic" it would have a base power of 90, however if it used the move "Dark Pulse" It would technically have a base power of 120, even though it normally has a base power of 80. In addition to STAB, some types are immune to some status moves, even though they aren't immune to the type of the status move, and vice versa. For example a Venusaur is immune to moves such as spore, sleep powder, and leech seed, because those are grass type status moves, but it isn't immune to grass type. An example of the opposite of this is the move Hypnosis, working on any dark type Pokemon, even though it's a psychic type move.
Where to play Pokemon
You can either play on the website https://play.pokemonshowdown.com/, or you could buy a Pokemon game. Playing one of the actual games on your DS, or 3DS is fun, although it takes a lot more work to make your team, and Pokemon Showdown is a really fun website for beginners, and experts alike.
All in all, there are a lot of things that go into Pokemon battling that make it an interesting, and fun past time.