PSA to all new players: Start working on your medals as soon as possible

If you plan on taking this game somewhat seriously you're going to have to work on your medals sooner or later. These tasks are tedious and require a bunch of time spent on the game to complete them.
Here are some tips on where to start:

The type collector medals: this will come with the time, but someone may be harder than others like dragon -type or steel-type. Take advantage of community days and Spotlight hours, and use the mysterious box for steel-type by catching meltan.

Evolution medal: there are some Pokémon game that have no use case except for being dex entries (raticate, kakuna, etc). There's no point saving candy for these, so evolve and transfer them.

Berry master, Gym Leader, Battle Girl: these medals go hand-in-hand. Defeat as many gyms as possible, play a pokemon, and feed the berries remotely. You can do this by scrolling down on your daily research tasks and clicking on the pokémon.
I would advise to mostly use razz berries and nanab berries, since they are mostly useless outside of this use case. This also gives some free Stardust!

Successor: for this one you need to mega evolve a Pokémon 1000 times. The easiest way is to get two mega Beedrills with a high mega level and simply keep evolving them one after another. It's tedious but it is the fastest method.

If you want any other tips please let me know and I'll try to provide some for you!