To say you're willing to learn on a CV, use words like 'curious', 'ambitious', and 'eager to learn' to describe yourself in your CV personal statement. Words like this will make it clear to employers that you're looking forward to learning on the job.
You can also show some proof of your willingness to learn. For example, add any qualifications you have that show you have learnt something new outside of your primary job or studies. For example, if you studied German at university, but took a class in drawing in your free time, it might be worth mentioning to showcase your willingness to learn.