I have a few questions about this:
I was reading an article about grammar, and on it they used the example:

"Ich muss dir was erzählen."

•Why didn't they use "etwas" instead of "was"? I thought "was" means "what".

•Also, what is the difference between "erzählen" and "sagen"?

Question

I have a few questions about this:
I was reading an article about grammar, and on it they used the example:

"Ich muss dir was erzählen."

•Why didn't they use "etwas" instead of "was"?  I thought "was" means "what".

•Also, what is the difference between "erzählen" and "sagen"?

Answer

"was" can be used as a colloquial short form of "etwas". 
And what is the difference between "to tell"  and "to say" in English?

Answer

Is it correct to "say a story" in English?

erzählen = narrate
sagen = say