Don't defrag SSDs. You'll wear out the drive faster. And OS X, like Linux distros, doesn't need to defrag, as the file system organizes things automatically.
In terms of clean-up, you can install clean-up apps like Disk Diag and many others. However, some can do more harm than good and I suggest you just leave it be. The only maintenance you need to do is to delete rarely used documents, movies, songs, etc.
How is the MacBook starting to get slow? The SSD is the fastest currently of any laptop and the hardware is top-notch. And you have 16 GB of RAM! (As you can see in my signature, I have the same laptop and in my experience, is very fast for my use.)