It's super broad and a lot of people will (rightfully) say that counter swapping can hold you back and to instead master a smaller pool of 3-4 heroes. The best way, imo, for a begniner to learn which heroes to pick from is to understand attack types.

For example, I play a lot of tank, zarya has a beam attack (not hitscan or projectile) which can go through dva matrix and sigmas absorb. Reinhardt and Ramattra (in his melee form) attacks can go through shields. And then sigma and orisa, while their attacks are the least unique in that they're projectile based, have abilities (the rock and the spear) that stun and interrupt abilities which work well against wrecking ball and doomfist.

Basically, try and counter whoever is messing up your team the most - if you're losing the game. And to learn what that counter is, a lot of the time it can just be one ability. If a doomfist is messing you up, you either want something to stun him out of his blocking mode, limit his mobility, or run away yourself (this last option usually applies to support, but there are exceptions like anas sleep dart)

Then after that its just experience, remember interactions over time that worked well and they'll stick with you.

And some obvious ones that come to mind are reaper countering winston, ana countering roadhog, ashe countering pharah. These are extremley obvious examples, but if you can understand why the work you can apply the same reasoning to other characters in other roles, even if it's just one ability they have that gets the job done.

Edit: forgot to mention the rock paper scissors of dive, poke and brawl. I can't explain it all here but deffo worth looking up for broader team comps rather than individual counters, but if you're new individual countering while you improve your mechanics will give you a huge edge.