您贵姓？
May I ask your surname?

我姓李，我叫李明。

Two verbs, two jobs. 姓 xìng takes the family name and nothing else — 我姓李. 叫 jiào takes the whole name or the given name — 我叫李明, and 张小美 could say 我叫小美. So 我姓李明 is wrong, and so is 我叫李 on its own. This is why a Chinese self-introduction so often comes in two beats: surname first, then the full name.

请问，您贵姓？

请 qǐng before a verb is the polite frame of Chinese, so 请 + 问 'ask' gives 请问 — 'excuse me, may I ask'. 贵姓 is the honour you pay someone else: 贵 means 'honourable', and the phrase goes only with 您. You never say it of yourself, so the answer is 我姓王, not 我贵姓王.

王先生，您好！张小姐，您好！

A title follows the surname in Chinese, the mirror image of English: 王先生 is Mr Wang, 张小姐 is Miss Zhang. Listen to the second syllables — they are not alike. 先生 goes light and toneless, xiānsheng, while 小姐 keeps its third tone, xiǎojiě, two third tones in a row exactly like 你好. And when you use a title, 您 comes with it almost automatically: the title and the pronoun are doing the same polite work.

请问，您贵姓？我姓……，我叫……

Say the whole polite exchange aloud. Ask 请问，您贵姓？ then answer for yourself in two beats: 我姓 … first, 我叫 … second. Finish by greeting the person with their surname plus a title — 王先生，您好！or 张小姐，您好！