Here's a song you probably have heard before to get you started. Don't worry just yet about your rhythm, it's not really a rhythmic song. Just strum and sing your way through it - and if it really is your first song, congratulations!
Have a good time with it. There's no rush. You might find you're fumbling for some of the chords, or not remembering them and having to look back over to the diagrams on the left, but that's okay. Remember that until just now, you've been learning how to operate the various parts of the ukulele - this is a whole bunch of things at once!
Try it a few times, till you get tired of it. Then we'll try another song.