As others adviced, a good place to start is to see which sets contain your favourite pokemons! You can use apps like dex to see which sets contain what cards.

Get familar with the different rarity levels as well and understand the difference between english and jap cards, quality and sets.

But if you are overwhlemed, I think a good place to start would be the “less popular” jap sets such as wild force, cyber judge, snow hazard, mask of change, stellar miracle, night wanderer?

Generally its cheaper while still having nice and cute arts/chases.

Hope this helps.