Willing, ready and prepared. These are all good characteristics for volunteers, but as usual with the English language it's always good to be ready and prepared for words to have more than one meaning or sense. To be willing is fairly straightforward, because it's based on the word "will" meaning "vôľa" (NOT the future - check the link with the German "ich will", which means "I want"), so willing means "ochotný/á" and willingness is "ochota".

Ready and prepared both mean "pripravený/á" ("hotový/á" usually means "finished" or "done"), but they can also mean "ochotný/á", especially "prepared" - I'm prepared to give you a second chance means I'm willing to give ... But then "readiness" can mean something like "ochota" (We appreciate your readiness to help), as well as "being ready" (Rescue services are in a state of readiness), while "preparedness" usually means just "pripravenosť".