“I’m down for that” vs. “I’m up for that”

What does it mean when someone says "I’m down for that" versus "I’m up for that"?

What’s the real difference between ‘say,’ ‘tell,’ and ‘speak’?

What’s the real difference between ‘say,’ ‘tell,’ and ‘speak’? I often get confused about when to use each of them in sentences. For example, sometimes people say ‘He said something,’ and other times ‘He told me something,’ or ‘He spoke to me.’ They all seem to mean ‘to talk,’ but the grammar and meaning change. Could you explain how to use them correctly with examples?