If you're looking for structure, then determine a style you want to master and work your way through a book. For example: I'm interested in bluegrass, so I got Fred Sokolow's Jumpin' Jim's Ukulele Songbook. It begins with three ways to approach a bluegrass song: strumming with a thumb lead, strumming with an index lead, and rolling (Scruggs style). The book then takes you from the simplest of songs through to the most difficult and with each you're improving your skills in a systematic way. Many such books have a CD included, as this one does, and you can listen to how the song is "supposed" to be played while you work on it. Make your way through an entire book and you'll have come a long way not only in bluegrass, but in improving your rhythm, dexterity, and knowledge of chords. Then you can move on to a more difficult book in the same vein, or take up a different style like blues or Hawaiian. Some of your skills will apply, others won't, but you'll get better over time.