'to say' and 'to tell' are used in reported speech.

'to speak' and 'to talk' are used in direct speech.

reallifeglobal.com/how-to-use-say-tell-talk-speak

I should say 'thank you Anastasia' for this link.

examples

I speak English (you cant say "I talk/say English")

I was speaking to my mother last night.

I was talking to my mother last night.

I want to speak with you about want happened yesterday.

(speak with or to somebody

We talked about the weather

We spoke about the weather

She said she thinks it will rain today

She told me she thinks it will rain today

The red sign says STOP