Hit confirming will come with repetition and practice. Just comboing near someone is not necessarily a bad thing, because it puts them under pressure (this is called a block string). Begin consciously adding Lows and Highs to your block strings to increase your chances of landing an actual hit.
Highs are a big part of the game, but they're intentionally slow and rare. Look at Ragna's 6B for instance. He can still cancel into other attacks from it, so it's perfectly safe and he can just use it during his pressure. However, he can only gatling (cancel into) it from 5A, 2A, or 2B. None of those attacks have very much blockstun at all (the amount of time the opponent can't move after blocking a hit). This intentionally forces him to have a gap in his pressure (a frame gap) if he wants to use his High-hitting 6B, which is generally more likely to hit than a Low. This makes him vulnerable while he's preparing his 6B (6B's "start-up").
This are some common terms you'll see a lot on here.
As for punishing, I can give no better example than Ragna's C-Inferno Divider (623C). If you successfully block it, he's punishable for a long time afterwards during C-Inferno Divider's recovery (the time it takes after performing a move until you are able to move/block again.)