Everybody when they start learning ukulele can have trouble strumming and singing at the same time - it’s like rubbing your stomach and tapping your head! But with slow practice on your own and playing along with others it WILL come! You can find lots of videos on YouTube that will help. Just google “strum and sing” or “play and sing”. Here's a couple of good ones I found!
Beginner - Strum and sing at the same time
Video tutorial about how to sing and strum at the same time