There's no magic bullet for learning how to play ukulele and sing at the same time. You need to coordinate your left hand for chord transitions, your right for strumming, and your voice for the melody. This takes a lot of practice.
Make sure you're playing and singing in the same key. This requires finding the starting note for singing the song. Eventually, you’ll be able to do so by ear - after playing a few chords in the song, you’ll just “know” what to sing. However, if you can’t do this yet, try matching a recording.
Here are some tips: