In a word- everything that is even arguably relevant to your claims. The reasons are simple.
First, your attorney cannot do their best job for you without knowing the facts, both good and bad. They can frame the claims and the narrative to mitigate or minimize any problematic facts, but only if they know about them.
Second, if you are withholding information from your attorney, it is usually information somebody else already knows- an email with the opposing party, for example. Your attorney can only protect you from risks they know about.
Finally, the discovery rules require that you eventually disclose this to the other side on request, which means you gain nothing at all by withholding it from your counsel early in the process.
Learn more here about discovery in a civil lawsuit.