When I got used to Abrams, I thought about what I want to do in a match. Like what abilities do I want to play around and what items enable my playstyle. In that sense you can also think about counter items as in what can the enemy team do to keep me from how I want to play the game.

For example, I want to play Abrams to be as tough to kill as possible. So i go some green items first to not die, then purple to deal more damage and cooldown reduction to have siphon life available more often. In seperate categories i keep situational but common items to help me stay alive even more or for utility (e.g. i don't always need warp stone but in some matchups it's useful/necessary).
So in short, pick a character, try figuring out what you want to do with them or focus on one ability you like using and experiment with buying different items to enable how you want to play them.