Now all you need to do is practice moving between your chords, and add a few more to your repertoire. Don't stress! I will give you a clue on the easy ones you probably want to learn first!
Since the most common 4 chords do a LOT of songs, and you already know two, lets try these four.
Practice moving between the chords (with and without strumming) until you can do it fairly easily. G is a bit of a hard shape to get, but its worth while getting it.
From here on in, there is less learning, and more practice. I found i could sit and watch TV whilst moving my fingers between chords without strumming. Once i got to the point of being able to move between those four, without that tell tale gap, i would add a new chord in.
Once you have those 4 above, try adding the next four. These are all fairly common and easy chords, practice being able to change between the chords faster at this point, and memorizing them.
You have those four down pat? Excellent! here is my last few that are handy common chords.
About now you can probably play fairly well, in fact you should be heading over to http://ukulelehunt.com/tab-chords/ to download some music . Make sure you look under the 'Chords', not the Tabs just yet! Best part about this website is that you can find things based on your ability, start at the easy and move to the harder !
Next -> Step 6 - Different Strums