Use erst when something is happening (or has happened) later than expected. Use schon when something has happened earlier than expected.
Erst & schon are both adverbs and seem to be used in very similar contexts. Luckily, the differences between the two words are actually pretty straightforward.
The difference between erst & schon
1. How to use "erst"
The contexts where students usually get confused when it's about "erst" vs. "schon" have to do with time:
In these contexts, "erst" is often translated with "only" in English. This makes sense for a lot of situations, but be careful to not just memorize this without understanding the context.
You need to understand the intention behind using "erst": "Erst" expresses that X has happened or is going to happen later than expected:
Here, Peter is getting a bit nervous, because he hadn't expected to only be at exercise no. 2 by this point in time.
2. How to use "schon"
The adverb "schon" takes on the opposite meaning of "erst":
In English we would most often translate "schon" in these contexts with "already":