You can use these questions and sentences before saying the exact favor you want.
Formal & Informal
Do you think you would be able to do me a favor?
Would you be able to do me a favor?
Can I ask you to do me a favor?
Could you do me a favor?
Can you do me a favor?
I need to ask a favor.
I need a favor.
Can you do me a solid?
Positive Responses
Of course. What is it?
I’d be happy to help.
I’d be glad to help.
It’s my pleasure.
Sure, what is it?
No problem.