how could I install ROS on ubuntu 18?

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As listed at the top of the Kinetic install instructions, ROS Kinetic only supports Ubuntu Wily (15.10) and Xenial (16.04). If you need to run ROS Kinetic on 18.04, you will need to build Kinetic from source (which is not likely to work without some effort). Alternatively you could switch to a version of ROS that is supported by 18.04, such as Melodic.

You could also investigate running Kinetic on 18.04 in a Docker container if you are comfortable with containers.

The distribution schedule and links to pages describing compatibility between Ubuntu versions is listed on the Distributions page.