The old classroom rule is simple: use like before a noun or noun phrase, and use as before a role, function, or full clause. It is not the whole story, but it is still a useful starting point.
For example, say She sings like her mother. Here, like is followed by a noun phrase, her mother. But say As your friend, I should be honest. Here, as shows a role. You also use as before a clause, as in As I expected, the train was late.
- Like compares one thing with another: It looks like rain.
- As can mean in the role of: She works as a designer.
- As can also introduce a clause: Do as I do.
In everyday speech, many native speakers say like I said, even though more formal grammar traditionally prefers as I said. So the old rule is best treated as a guide, especially in careful writing, not as a complete ban on every other pattern.
