How do you invest in abandon houses?

Your question is very general and vague.

There really is no such thing as an "abandoned" house. Every piece of property is owned by someone. It may be vacant, and have been vacant for ages, but someone still owns it. (Bank, private party, estate, govt. entity, etc.) You would contact the rightful owner and negotiate a sale. There are tons of tactics for doing so, but it all depends on the specifics.

Yes, there is no such thing as a true abondaned house, there may be empty ones, run down ones, but someone owns everything, it may be the state or county or the owners may be lost but they are owned.

So you go to the court house and find the listed owner, contact them and try and buy the property

Abandoned houses can be the best properties to invest in, and often you'll find they are about to enter a tax sale foreclosure process if they have been left to sit long enough.

As the previous poster said, get their name and address from the county and try to contact them. Many owners of abandoned properties practically give them away.