'Just now' means 'a short moment ago'. It is a finished time in the past so past simple is most appropriate. e.g. I saw her getting in her car just now. 'Just now' can also mean 'right at this moment ', so with this sense you'd use present simple or present continuous. e.g. I'm busy just now so I'll help you later.

Simple past is best. Present perfect can be used. You might be able to use present but other expressions work better. "Just now I saw a rabbit run across the yard" "Just now a rabbit has come into the yard" (present perfect best because it's still there) "Look! Right now there's a rabbit running across the yard"