Never ever apologize for your speaking abilities. Even if you have only the speaking ability of a terrified mouse, you'll just increase your suffering.
Apologies can take many forms
An apology can be verbal: 'Sorry, I'm not a public speaker. I'll keep it as short as possible.' Or it can be simply an expression on your face that conveys a similar feeling. You might argue that it's okay because you're just building rapport. You're being honest and humble - which, surely, are virtues. Aren't they?
What audiences think
No. Audiences hate it. They groan internally, partly because the I'm-no-good apology is ingratiating, partly because it conveys fear and submission. It says:
It's also self-fulfilling. It paints you into an even tighter corner, constraining the abilities you do have.
This witticism from Kin Hubbard: "Why don't the feller who says, 'I'm not a speech maker' let it go at that instead o' givin' a demonstration."
Well, that's cynical humour. So let's change it to this.
Have the courage to speak without apology
If you're nervous, make yourself speak anyway - with no reference to your lack of ability. If you're nervous, the audience will see it and credit you for your courage in the face of fear.
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