If you are trying to learn how to "Improvise" or play lead parts on ukulele your first step should be learning the pentatonic scales. Playing music involves both a mental and physical component. To improvise you will need both the mental and physical components working in harmony however, the physical component must be mastered first. Don't worry about the music theory involved or even how to use the scales. Start by learning the shapes.
Click here to download a PDF that will teach you all five pentatonic scales for the ukulele. The PDF was created by a company called "Step 1". The scales can be played on any size ukulele. Although not necessary, I highly suggest that you use a "low G" string. Treat the scales as if they are simply part of a finger warm up regiment. Don't worry about anything but learning the shapes. Master them one at a time.
Once you have mastered a shape you can begin using it to improvise. All shapes in the packet is presented in the keys of C major/A minor. Without getting to deep into theory you just need to know that all of shapes can be used with songs in the key of C major or A minor. To get started improvising try using the notes of shape #1 in combination with techniques like slides, bends, hammer-ons and pull-offs. You can jam along with any of the videos below.
Good luck and happy jamming!