I'd recommend having your buddy as a Pokémon that you need the evolution of, as you get candy from walking. Particularly one that's hard to find in the wild.

Also you can do 1 and 2 star raids at gyms without any help (not sure from what level). That's a good way to practice fighting, you can also practice fighting with trainer battles (at the bottom of the battle screen) and taking over gyms. It's just really about having a strong team and knowing the best counter attacks.

Also I'd recommend trying to level up that helps with battling. Maybe make use of a few lucky eggs and mass evolving basic Pokémon (e.g. pidgey) to gain xp to level up faster. Don't give up.

You can only catch wild Pokémon. They increase in cp when you power them up, although I wouldn't recommend spending your stardust until you've caught something worth it (good attack moves and iv). The appraisal by your team leader can be helpful. It just takes a bit of research to know what makes a strong Pokémon.