Most trainers aim to become the best within their Pokemon Leagues. In the Hoenn Region, Professor Birch is the appointed professor who helps provide starter pokemon to the children and trainers first setting out on their pokemon journeys. What he provides is different in every place, but when you visit his home lab in Littleroot Town, he has a selection of Treeko, Torchic, and Mudkip for new trainers to choose from.
Pokemon Gyms are a way for the Pokemon League to test for worthy trainers. Any trainer who obtains 8 Gym badges by defeating the respective Gym Leaders may challenge the Elite Four and their leader, the Pokemon Champion (of Hoenn in this case, as each region as its own champion). Anyone who successfully completes the battle gauntlet of all 5 trainers (the Elite Four and the Champion) becomes the new Champion.
Gym leaders are fully realized trainers with full teams of fully evolved pokemon. but due to the nature of the challenge, if a trainer who has no badges shows up at their gym to begin their journey, the gym leader will choose weaker pokemon to use to even the odds. the more badges a challenge has, the harder the gym leader will battle, using more stronger and better trained pokemon depending on the number of badges the challenger has already accrued. This allows anyone to challenge any gym in any order for the purpose of badge collection and still expect a fair but difficult test everytime.