There's two ways to lower your action. You can sand the nut and/or the saddle down.
When fretting a string at the 3rd fret, you should be able to slide a business card between the string and the top (crown) of the first fret with a little bit of friction.
If there's no friction, then you should lower the action at the nut.
Take the strings off and remove the nut.
Get some 220 grit sandpaper, and nail it to a small, flat piece of wood or something. It needs to be flat, whatever it is.
Run the bottom of the nut over the sandpaper a little at a time, checking your measurements frequently. You'll have to restring the uke a few times and check your measurements until you've got it just right.
If you can't remove the nut, you can also file the slots in the nut a little at a time. Be careful if you choose this route. If the slots get too wide, you can have problems.
Now restring your uke and check the action at the 12th fret. The space between the string and the crown of the 12th fret should be between 1/8 and 3/16 of an inch.
If it's too high, you'll have to adjust the action at the saddle.
To lower the action at the saddle, calculate the difference between your action at the 12th fret and 3/16 of an inch. Loosen your strings, take out your saddle, and mark this difference on it with a pencil, starting from the bottom of the saddle. Sand the bottom of your saddle down to that pencil mark.
Hope that helps. Good luck.