In Pokemon Ruby and Sapphire there are three starters. Torchic, Treeko, and Mudkip. I advise to go with either Torchic or Mudkip as it is hard to find a good fire and water Pokemon in Ruby or Sapphire until you get Groudon or Kyogre. Mudkip evolves into Marshtomp then into Swampert, Treeko evolves into Grovyle then into Sceptile, and Torchic evolves into Combusken then into Blaziken.
Torchic is a fire type starter Pokemon, or in Japanese it is known as Achamo. Its ability is Blaze, that is where it raises the power of Torchic’s Fire-type attacks by 1.5x while Torchic has less than 1/3 of its max HP (rounded down). It can learn moves as strong as flamethrower, firespin, and peck. Torchic evolves into Combusken at level 16 and then into Blaziken at level 36. Its weaknesses are water and ground Pokemon.
Mudkip is a water type starter, or in Japanese it is known as Mizugorou. Its ability is Torrent. That is where it raises the power of Mudkip’s Water-type attacks by 1.5x while Mudkip has less than 1/3 of its max HP (rounded down). Kind of the same as Torchic’s ability. It can learn moves as strong as surf, ice beam, and hydro pump. Mudkip evolves into Marshtomp at level 16 and then into Swampert at level 36. Its weaknesses are electric Pokemon.
Treeko is a grass type and is known as Kimori in Japanese. Its ability is Overgrow, which is where it raises the power of Treecko’s Grass-type attacks by 1.5x while Treecko has less than 1/3 of its max HP (rounded down). It can learn moves as strong as giga drain, mega drain, and energy ball. Its weaknesses are ice, fire, and flying Pokemon.
In this game you are moving into a little town of Littleroot. Where you will battle other trainers to gain experience to level your Pokemon up. You then have to gather all the gym badges and challenge the Elite Four. But before that you must stop either Team Magma or Team Aqua which depends on which game you are playing. You will also battle the legendary Pokemon Groudon and Kyogre which also depends on which game you are playing. You may catch it or you may defeat it. Good luck with your game play!