Hold your pick between your thumb and first finger. When you use a pick, you can pick one string at a time, or you can strum up and down. When you strum a ukulele with a pick, you don't have to worry about what to do with all of your fingers, you can simply strum up and down.
Is it okay to play ukulele with a pick?
it is perfectly fine to use a pick for a different sound. More players today are playing melodies on the uke, or adapting rock songs and other styles to the uke. So if a song or style demands a pick, then by all means go for it! Ukuleles have nylon strings, so you won't need a super heavy pick.