a friend of mine founded a start up and asked me to create an android app for him. I did it with java in android studio but since i never got in contact with commercial use before i want to know what i have to be aware of. Like if im selling the app to him (without publishing it on google play) do i need to buy a java license? Do my friend has to buy the license too? And do i need to put my app under a own license of me?
You don't need to buy a Java license.
As per the android docs about JDK (here), they bundle a copy of OpenJDK.
OpenJDK is under the permissive GNU General Public License version 2 (GPL v2, link), which allows for commercial use at no cost.
As long as you meet the license requirements of any other packages/tools in your app (like SDKs that you integrate), you should be OK from the licensing point of view.
Other matters (e.g. cost your friend pays for the app, how you license or sell the app to your friend formally etc.) will need to be clarified between you and them - we are not part of those discussions and are unable to give you accurate advice on that.