When I first started I just bought any cool card that I found. Usually these were Pokemon and Yu-Gi-Oh cards.
These were the hot cards at the time, and in fairness still are.
If I came across one that I liked and it was affordable, I would buy it without a goal in mind.
I just kept buying.
Things really picked up once I only focused on my goals (Pikachu collection, etc.). I did still occasionally buy other cards that I thought were cool-looking, but I tried to keep that to an absolute minimum.
Collecting Pikachus was already pricey enough.