Good advice above.
I will add this survey I did of the most popular breeds among my selection of battlers
https://pastebin.com/UE8TSGGsFrom that, you can see, for example, that nearly everyone favours the S/S Flayer Youngling, but people have a spread of Sky-Bos. I remember reading it originally and saying that you would not regret picking up the most popular breed in each case. (Careful of the duplicate moveset cases like Rabbits though.)
Have a look down the list, and do please come back with any specific questions.

Also, of all things, DON'T BE AFRAID to PvP. It is the most anonymous thing you can do in the game. Your opponent sees you as a random NPC, just as you see him. Neither of you will ever know who the other is, unless one of you makes distinctive renames of your pets. You may end up not liking it, and you will probably lose a lot of your early matches, but there is no possible reason to be afraid of it. Just pretend the figure before you is a random tamer created by the game.
It's definitely worth doing in Pet Battle Week, when you get a quest for 5 wins, and lots of beginners are in the queue.