it's really just about knowing matchups. Different characters have different strings and more useful attacks, at different ranges. If a character has a fast startup string that advances and hits low, or maybe mid mid low, probably should block low. If they have a fast overhead, block standing if you're negative. If you're in a 50/50 situation where it's a raw guess as to which is coming, you have to read the other player's tendencies, or try and fuzzy guard if possible (depends on which move comes out faster, if it's the low, block low quickly and release down while holding the block that way if the overhead comes, you're covered), or lastly decide which option is more damaging.
For example, if the opponent has both oh's and lows, you have to know that maybe the OH launches and leads to full combo where maybe the low just does 7% damage or something, in which case, you'd rather eat the low, so block standing.
Honestly though, this game doesn't reward blocking very much at all, and if you end up blocking a lot, you will get chipped out, thrown, will guess wrong and eat big damage, etc. Really have to know how to use your armor options, pokes, reversals, wakeups, etc. The biggest thing comes back to match up knowledge and knowing what the other player can do in various situations, and what you can do in response in terms of frames.
Short answer: there's no simple answer on how to block in this game, it's matchup dependent.
One other general tip though if you're new: If you don't have meter to wakeup, vary your wakeups with delay wakeups, tech rolls, and just stand up. If you don't have meter or don't see that they're whiffing a meaty or something, BLOCK! It sounds simple, but so many people new wakeup with buttons and it's a quick way to get bodied free.
