Formal:
* 我 会 说 普通话。 (Wǒ huì shuō pǔtōnghuà.) - This is the most common and formal way to say it. It literally translates to "I can speak Mandarin."
* 我 会 说 中文。 (Wǒ huì shuō zhōngwén.) - This is also formal and means "I can speak Chinese." It's more general, but can also be used to specifically mean Mandarin.
Informal:
* 我会说普通话。 (Wǒ huì shuō pǔtōnghuà.) - This is still considered polite, but a bit more casual.
* 我讲普通话。 (Wǒ jiǎng pǔtōnghuà.) - This is a more informal way to say "I speak Mandarin."
* 我会讲普通话。 (Wǒ huì jiǎng pǔtōnghuà.) - This is also an informal way to say "I speak Mandarin."
Note:
* "普通话" (pǔtōnghuà) is the official standard spoken language of China, also known as Mandarin.
* "中文" (zhōngwén) is the term for the Chinese language as a whole.
It's generally best to stick to the formal ways of saying "I speak Mandarin" unless you are very comfortable with the person you are speaking to.