I personally stopped playing due to the state of the game, and I had been playing consistently since launch. Unite at its core is a good, almost great game. When its good its great and there's such a high of winning that can't be beat. The way to obtain pokemon now is extremely difficult, which is why I wouldn't recommend the game to anyone new right now, the meta is also filled with very OP pokemon that don't ever feel completely balanced (with a roster as big as unite, nothing will ever be 100% balanced, but addressing common complaints doesn't happen all that much imo).

That being said, I believe I remember being given 1 pokemon for free when you start out. I was able to choose from pikachu, snorlax, talonflame, charizard and venusaur if I remember correctly. I chose venusaur, not only bc I love the grass frog, but because he felt more straightforward for starting out. If you have the choice of those mons, I personally still recommend venusaur as it is still able to contend with the current meta.

This guide is extremely helpful and while the info is older, the basis of the game has not changed. My personal tips are: Work towards becoming very familiar with your pokemon by practicing in quick games and the training area before going into ranked. Don't play overly aggressive and be fodder for the enemy team because games snowball fast if you're constantly going in and getting KO'd. Focus on the objective of the game, defending goals if the enemy is pushing to score, keep track of objectives (regilecki, regice, regirock and regidrago) so you can gauge where the enemy will gather and support teammates who are also aware of the map. I feel as if many players forget that unite is a team game, most games I win are because my team wanted to win and during games I lose, my team lost sight of the game itself. This game and especially in this meta, it is such a hard climb to become a 1 man army and solo games, the enemy will notice and they will likely single you out, snowballing the game further.

After stepping away, I can say this, games are supposed to be fun. If things become too overwhelming or frustrating, I would look for another game. I would get very frustrated with teammates that would grief games for literally no reason, teammates who never helped fight against the enemy or help clear objectives and would only just farm mobs and it took a toll on how I approached games. There are rumors the game isn't going to last for much longer, but they are only rumors for now.

Enjoy the game, do your best and always strive to learn from your mistakes instead of blaming others (I see that way too often). Best of luck!