Just keep buying/farming shards for those characters. Don't stop until they're 7*. This is true for any character that you farm. It's not worth it to just add one star here and there - 7* or bust. It takes a while, but it's worth it if you're enjoying the game.
You might look around you tube - there are new player guides out there. Try to find one that's less than a year old. If you can't, just find the most recent one you can find from an established content creator and begin to lay out a path to your first few legendaries (Like GMY). Watch a couple of them even - from different sources. Different people have different ideas on what's the best way to get started - best to hear more than one take.